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Need Help Few Issues With 2001 Corolla

by sxd, October 8, 2010



I searched the forum, AND talked to some mechanics and haven't gotten any real answers. I believe these are all different issues but they may not be.

I bought a used 2001 corolla last year with about 180k on it and it had a check engine light on. On my initial read from a used car inspection, the mechanics told me that everything was fine and the check engine light was just saying the CAT needed to be replaced.

I bought the car, went to a place that changes cats and the guy told me he didn't want to change it because he has no idea if it's actually the cat so i shouldn't waste my money and to take it to a mechanic and see if that is even the problem at all.

I had since taken it to three mechanics and they ALL are telling me different stories. One says it's probably the O2 sensors bad or dirty, another says it could be anything so just start changing the cheaper parts first till the problem is fixed, and another says the check engine light will possibly never go out even if you change everything that is realted to that code. .... ?!!! is there a tool or meter of some sort to check the O2 sensors? aren't they electronic?......

well, that was the first problem.

second, the car does that studdering thing people are describing but i believe it's a little different on mine. This happens very rarely but it really bothers me when I'm in the middle of an intersection trying to turn left. basically, from a stop at a red light or while waiting to turn or get into a moving lane, SOMETIMES the car will not move. I step on the accelerator, and there is nothing. The car doesn't move, the RPM gauge doesn't move, the engine doesn't make any noise... literally nothing. I can sometimes avoid this by turning off the overdrive but it doesn't always work. Like someone else mentioned, it's usually when i'm going from stop to half throttle, but it's done it sometimes even when i'm pressing really light on the gas. it does this maybe once a week and i drive really carefully now. make sure i have enough space between me and the travelling car should mine not move. when it finally does move there is a little studdering before it get's back up to speed. it does this on hills as well so i try to make sure the overdrive is off when i approach hills. this happens mostly when the car is warm, i actually don't ever remember it happening when it was cold. and it never does this in park.

third, my oil went almost completely dry, and it had been about three months overdue for an oil change, but the last time i went six months, it was only low by a quart. this time it was low by about 2 quarts. I only knew this because i had to accelerate out really quick onto a busy street and it made this really loud metallic whirring sound, almost like a car with a timing chain instead of belt (which i noticed this car kind of sounds like anyway... or maybe my pulley bearings are bad...??) and the oil light went on so I immediately knew what it was and turned into a gas station and put in some oil. I use 5-30 syntec but they only had synthetic mobil1 so i filled with that. I remember one of the first cars I had I was using royal purple and i didn't know there was a gasket leak, was running the car dry for months, so i'll check my oil every month now to see if there is a problem beyond normal consumption. but i'm wondering if i should switch to higher mileage oil now and see if there is a difference in rate of use.

fourth, the most recent problem is there is a not loud but noticeable whining, vibrating, noise, almost like an industrial vent fan that needs to be lubricated (i work at a restaurant). It's coming from the engine compartment but I can't tell where cause this engine is so damn noisy compared to my old 95 corolla. I know the power steering noise and it's not that because one, it sounds different, and two, the noise goes away when I apply pressure to the wheel in either direction. goes away when i turn it, comes back when i let go of the wheel. power steering and belt issues work opposite of that right? noise when you turn the wheel?... I am assuming this is an easily identifiable problem but the shop I called can't give me any info and they want me to just bring it in for them to "take a look at"... really? it's opposite of the usual power steering leak noise. You have NO IDEA what it could be? I can't search for this cause all I get is power steering issues in my search results. I also just noticed that the whining pitch gets lower when I accelerate but I cant tell how much lower cause there's engine noise and wind noise when I accelerate so...

please help. I have a baby and i'm wondering if i should just get another car as soon as i pay this one off.

Sounds like this car had the heck beat out of it and was poorly maintained by the previous owner(s). I hope you didn't pay too much for it.

I searched the forum, AND talked to some mechanics and haven't gotten any real answers. I believe these are all different issues but they may not be.

 

I bought a used 2001 corolla last year with about 180k on it and it had a check engine light on. On my initial read from a used car inspection, the mechanics told me that everything was fine and the check engine light was just saying the CAT needed to be replaced.

I bought the car, went to a place that changes cats and the guy told me he didn't want to change it because he has no idea if it's actually the cat so i shouldn't waste my money and to take it to a mechanic and see if that is even the problem at all.

I had since taken it to three mechanics and they ALL are telling me different stories. One says it's probably the O2 sensors bad or dirty, another says it could be anything so just start changing the cheaper parts first till the problem is fixed, and another says the check engine light will possibly never go out even if you change everything that is realted to that code. .... ?!!! is there a tool or meter of some sort to check the O2 sensors? aren't they electronic?...... 

Answer to this is pretty complicated. Actually - all of the mechanics/technicians are right on this problem. Especially on a P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)). On this generation of Corolla, the issue is very rarely with the catalytic converter itself. The problem may lie with an exhaust leak, or faulty O2 sensor(s), or bad engine coolant temperature sensor, to a faulty ECM. Even if you replace all those items, the CEL may still pop up from time to time.

Try this first. Reset the ECM and see if the code comes back. If it does, then have someone "backprobe" the O2 sensors. This is a surefire way to check and see if the O2 sensors are reading correctly or not, during this time, they should be checking for exhaust leaks. If everything check outs, then you can replace the catalytic converter. Even so, 60% or so, the CEL will come back. Most of it due to how the ECM interprets the signal from the downstream (post-cat) O2 sensor. In those cases, you'll have to run an O2 sensor defouler (moves the O2 sensor out of direct exhaust stream) and may "fix" the CEL.

second, the car does that studdering thing people are describing but i believe it's a little different on mine. This happens very rarely but it really bothers me when I'm in the middle of an intersection trying to turn left. basically, from a stop at a red light or while waiting to turn or get into a moving lane, SOMETIMES the car will not move. I step on the accelerator, and there is nothing. The car doesn't move, the RPM gauge doesn't move, the engine doesn't make any noise... literally nothing. I can sometimes avoid this by turning off the overdrive but it doesn't always work. Like someone else mentioned, it's usually when i'm going from stop to half throttle, but it's done it sometimes even when i'm pressing really light on the gas. it does this maybe once a week and i drive really carefully now. make sure i have enough space between me and the travelling car should mine not move. when it finally does  move there is a little studdering before it get's back up to speed. it does this on hills as well so i try to make sure the overdrive is off when i approach hills. this happens mostly when the car is warm, i actually don't ever remember it happening when it was cold. and it never does this in park.

When you step on the accelerator when it does this - you mention that car does "nothing". Nothing as in not moving, gauges not responding, etc. Not understanding the RPM not moving - does the engine sound increase? Engine shaking at this point?

What is happening here could be from a number of issues. Could be related to the oil consumption issue you are seeing in your third point. At this point, too many potential culprits to track down. If you haven't already gotten a comprehensive tuneup done (plugs, fluids, check for vacuum leaks, clean MAF, replace PCV, clean throttlebody, serpentine belt replacement, hoses, wear parts, etc.) - might want to get this done sooner or later. As Larry mentioned, could all be related to a maintenance issue.

third, my oil went almost completely dry, and it had been about three months overdue for an oil change, but the last time i went six months, it was only low by a quart. this time it was low by about 2 quarts. I only knew this because i had to accelerate out really quick onto a busy street and it made this really loud metallic whirring sound, almost like a car with a timing chain instead of belt (which i noticed this car kind of sounds like anyway... or maybe my pulley bearings are bad...??)  and the oil light went on so I immediately knew what it was and turned into a gas station and put in some oil. I use 5-30 syntec but they only had synthetic mobil1 so i filled with that. I remember one of the first cars I had I was using royal purple and i didn't know there was a gasket leak, was running the car dry for months, so i'll check my oil every month now to see if there is a problem beyond normal consumption. but i'm wondering if i should switch to higher mileage oil now and see if there is a difference in rate of use.

You really get into the habit of checking the oil level every time you fill up with gasoline. This way, you can track oil consumption more precisely, and catch periods when oil is running low and top off before it becomes a problem. Once you see that low oil light come on, there is a very good chance that engine damage has already occurred. Engine is fairly noise, even when it is running well, part of which is due to that it is running a timing chain vs a timing belt. Engine is also 20% smaller (physical packaging) and 23% lighter then the previous generation of engine while providing better fuel economy, power, and emissions. None of which are good for controlling noise. Many have reported good results of running synthetic and synthetic blends high mileage motor oils in this car. Though depending on your oil consumption levels, this is only a bandaid fix. Need to find the true source of the oil consumption. Compression test of the engine is highly recommended. If compression is not even or too low - you can stop right there. No "fix" in a can will help you at that point, now you will be looking at an engine rebuild or even a new car.

Excessive oil consumption would also cause a lot of the issues you are having here. Oil being expelled through the exhaust system can foul and damage engine and emission components. Valvetrain starved of fresh oil can cause the engine to be excessively noisy. Along with potentially damaging the VVTi system and causing low compression, would lead to performance issues. Key to seeing if this is something that can be turned around or if it is cheaper to dump the car now - you'd have to start tracking it often - checking the oil every day is not out of the question. Checking once a week is fine, at the very least, every time you fill up. Going a month between oil level checks could be death to an engine that is consuming any oil. Oil consumption can start slowly and quickly take off, need to stay on top of this to find out where you stand.

fourth, the most recent problem is there is a not loud but noticeable whining, vibrating, noise, almost like an industrial vent fan that needs to be lubricated (i work at a restaurant). It's coming from the engine compartment but I can't tell where cause this engine is so damn noisy compared to my old 95 corolla. I know the power steering noise and it's not that because one, it sounds different, and two, the noise goes away when I apply pressure to the wheel in either direction. goes away when i turn it, comes back when i let go of the wheel. power steering and belt issues work opposite of that right? noise when you turn the wheel?... I am assuming this is an easily identifiable problem but the shop I called can't give me any info and they want me to just bring it in for them to "take a look at"... really? it's opposite of the usual power steering leak noise. You have NO IDEA what it could be? I can't search for this cause all I get is power steering issues in my search results. I also just noticed that the whining pitch gets lower when I accelerate but I cant tell how much lower cause there's engine noise and wind noise when I accelerate so...

Again, a lot of potential culprits here. Anything from a worn serpentine belt, to bad pulley bearings, to worn belt tensioner. Loading the belt (by turning the steering wheel) and having the noise go away, sounds like you have a bad bearing somewhere in the loop. A quick check is to remove the belt and run the car (couple of minutes max) and see if you notice the noise or not. Again, this falls under the maintenance issue of the car - if the belt has not been replaced in some time, shows lots of heavy cracking, glazed or otherwise damaged - need to replace it.

yeah the engine sound DOES NOT increase and there is no shaking at all.

it's literally, like i'm not stepping on the pedal at all , and I don't understand it either because I am DEFINITELY stepping on the pedal.

I was thinking the throttle cable might be the problem or the the throttle itself (maybe stuck closed?) but when I check it from under the hood, everything seems fine....

yes i tried it with the engine warm and cold. I can't MAKE it do that, it just randomly happens.

also the car just started clicking/rattling a little on hill stops, hill turns etc, so... MAYBE the humming is related to the steering system. but, I've never owned this generation corolla so, how would I know.

I will take it in for a tune-up and get the steering system checked, and keep checking the oil.

and as soon as i get the money, I'm getting another AE 101 cause, I know EXACTLY how that car is supposed to sound and run.

I just realized I failed to mention...

in regards to the power loss issue.... it happens almost ALWAYS when the A/C is on. I haven't had that problem since my last post cause I haven't been using the A/C at all since it started getting cold... now i'm going into winter with the heater... with this new little piece of info, does anyone have any further insight into what that problem could be?

also... a few days after my last post, the car just stopped making the whining noise. the engine sounds fine. well... it's still a noisy engine but, it sounds like it did before i started having all these problems. and no i didn't do anything different, although i have been running premium fuel for about two months now.

is this like a terminal illness situation where there's a sense of euphoria right before the end and then my car just dies one day?... do engine's operate like that?.... I know it's silly but I've never had an engine die on me before. I've had them go bad but never die.

as a side note, the reason i switched to premium was because i thought maybe my fuel line or something was dirty and needed a good cleaning also remembering you get better mileage off of higher octane (I got REALLY good mileage when my oil started to go dry btw) but i'm just gonna assume cleaner gas had nothing to do with suppressing the problems since... that would just seem too easy. like i said, i kind of expect the power loss issues to return once i start using the heater.

Power loss when you kick on the A/C could point to a couple of things. One, the A/C compressor is locked up, so when the magnetic clutch engages on that pulley, it drags on the serpentine belt and causes power loss there (engine has to work harder to turn a stubborn pulley). The other is the idle-up circuit. When you kick on the A/C, the ECM will kick up the engine idle speed, to keep the idle steady when the A/C comes on. That system is interconnected to several VSV and the the fuel system (1ZZ-FE uses a returnless fuel system, fuel is supplied based on demand). If that circuit is not working correctly, possible that the system will not supply enough fuel to accelerate.

Higher octane fuel is just more resistant to detonation. Some premium fuels up the additives, but most are exactly the same as the regular grade (cleaning wise). If you have markedly higher fuel economy with the higher octane fuel, this could indicate some heavy internal deposits on the combustion chambers.

As for why the car stopped making that noise - hard to say. Could be weather related, could also be one of those "automagical" fixes. Not a good feeling, as these intermittent problems can be a bear to diagnose. From what you've seen so far, I wouldn't be surprised that this is a multiple culprit issue here. How extensive and if you need to dump the car for something else - I can't really say one way or another. Could be a very simple fix (ie, bad vacuum line or dirty throttle body), or could be more complex. Having it run dry of oil once or twice may have sealed the engine's fate. The 1ZZ-FE's timing chain is tough on oil, if it starts to consume a large amount of oil and you don't stay on top of it with oil top offs, usually end up with a paperweight in the end.

The whining noise may be from the timing chain, as opposed to quieter timing belts on earlier models. My `99 has always made a slight whining noise--the Toyota shop told me it was normal. When your oil level gets low the noise probably gets louder.

god damnnit!

alright first of all the noise came back and then went away again. went in to a mechanic who said it sounds, seems like the power steering pump is going out. it was rattling but it looked like the pulley issue to me that people on these forums bring up from time to time. but the mechanic said no, it's not.

the snow just about hit at that time and no shops had time to look at a car cause everyone was changing tires. so i couldn't take it anywhere. just drove it. then it stopped again like, the next day. this was a week or two ago.

no issues again as of yet.

but i just went to clear my codes cause i have to get an emissions inspection (that didn't work, they told me to drive a bunch and come back in a day or so), and santa brought me some brand new engine codes.

i didn't write them down cause i was just trying to get my emissions inspection. now they're all cleared and the monitors haven't run yet so i need to wait, but here's what i remember.

i still had the 420 code.

there was one that i think was 120 and another that was 130.

also read something about o2 sensor, and another about running lean.....

ok so prior to this, I filled up with premium, and a bottle of octane booster/cleaner... i think it was a 7 number increase. hyper brand or lucas, i really don't remember.

i also went and got a oil change 5-30 royal purple and a quart of lucas full syn oil stabilizer.

so.... what is my drive cycle?

i'd like to pass emissions without having to change something that MAYBE is the problem, unless anyone here can tell me an o2 sensor code is a little more accurate than the 420 and i can go ahead and have a mechanic "backprobe" etc. but are they even gonna do this? or area they gonna be like "we're gonna fix your car OUR way. change everything that you can afford to, and if that doesn't work THEN we'll backprobe"....

cause honestly, I've never had a mechanic even SUGGEST checking something. they always say they're just gonna change everything out.

so damn frustrating and yes, i'll be looking for a new car soon but i have 7 payments left on this one.

Just a note, when they say x number of octane points of increase on those octane boosters, that is a fraction of the actual octane number. Example - octane booster says will boost up to 7 points, filled with regular - 87 Octane, the resulting mix will be 87.7 octane. The main ingredient in that Lucas stuff is the same booster that they used in leaded racing gas - been debated if that will actually hurt the emissions system or not, some studies says yes (company that makes them), some says no (EPA). Take it as it is.

Once you reset the ECM to clear the code, the I/M readiness status (the part that the emissions testing is interested in) is set to a "not set" or "not ready" state. To get them to move to the "ready" state, you have to run through a drive cycle. Drive cycle is made up of different RPMs for a set time, periods of idling, start/stop cycles, etc. Varies from manufacturer to manufacturer, for some people and their driving conditions - might take a day to set to ready condition, for other - might take over a week and hundreds of miles.

There was a post on there some time ago that mentioned that they seemed to come up with a "recipe" to set the I/M readiness monitor.

- Start your vehicle, straight line as possible (highway) at 50 MPH for 10 minutes

- Stop your vehicle while in gear (not in neutral or park), let vehicle idle for 5 minutes

- Repeat above up to 5 times, check with a scan tool between each run to monitor the readiness status - do NOT reset the ECM or you'll have to start over

For the "lean" condition - I'd start by cleaning the MAF sensor and making sure that your airbox has a good seal (check the clips and tabs in the rear, make sure the top is secured).

P0420 could be just the ECM crying "wolf", note how often it comes back. If the car has been consuming a decent amount of oil for some time, there is a good chance that the O2 sensors have gotten fouled/turned lazy.

As for mechanics that can actually diagnose the issues you are seeing - few and far between. Mechanics cannot make money suggesting fixes, they make money by swapping parts and charge you labor, only way they can make any money.

NOTE: Not advocating this as a fix, but as a trick to get you to quickly pass emissions by avoiding the P0420 code is to run a spark plug defouler on the downstream (post cat) sensor. This will space it away from the exhaust stream and make it think it is reading a good waveform signal. You'll have to modify the defouler's end with a drill to make it work on the O2 sensor - lots of info on this "trick" online. A savvy emissions testing tech will also see this, some will let you slide, some will automatically fail you.

yeah, the booster said "numbers" not points.... still it could say whatever they want it to say, we just trust what it says on the package to be true right.

i went to auto zone and asked the guy if he could check if my system was "ready" yet, since i just cleared it the other day. then he goes " i just cleared it"... i'm thinking, great now i gotta drive a bunch again.

how much does it actually cost to replace the O2 sensors?... it looks like it's just unplug, unbolt, bolt, plug.... maybe some cleaning. should i just do this myself? it seems really simple.... if so, I just need to know which sensors are SUGGESTED to use, since there are a bunch of generic, universal, etc, parts floating around ebay and what not, but it looks to be relatively cheap....

i'm sure i could do the cat myself... just need to buy it right? unbolt, bolt...

im just asking about the O2's cause i've never touched anything on a car other than bolt on mods and audio systems... don't know if the O2 sensors are like halogen bulbs.

dooooonnt tooouch theee buuuuulllllb......

MAF sensor... do these just need to be cleaned or should i consider buying one as well?

the real issue here is, i need my emissions clearance so i can get my registration.

i'm in washington so it says i spend $150 on repairs by a certified emissions specialist, and i can get the emissions waived till the next time... then it's another $150 and i'd kind of like to get the problem fixed sooner rathar than have the "specialists" run $75 worth of diagnostics and doing $75 worth of work which pretty much equals they look at it with a scanner, they clean something, $150.... CEL is still on. see you in two years.

so if there is ANYTHING else it could be.... say i change the O2 sensors, the cat, clean the MAF sensor. clear, do a drive cycle, and I'm still pulling up a p420 (which was the original and ONLY code that was on the car when I bought it).... would that then mean, burning too much something as in a blown gasket? $400-$1000.... cause if that's the most likely case, i'll stop after the cat and sensors and get rid of the thing in 7 months.

-------------------------------------------------

also. I really miss my AE102. would have been way cheaper to ship it here (i'm from hawaii) and clean or change that damn EGR valve.

Yeah, those Autozone guys - interestingly, they are NOT supposed to erase/reset the ECM - as that is a liability issue. Assuming the guy just got punchy with the buttons - as you can just read the codes without erasing them.

O2 sensors are really plug and go, especially if you get the OEM-type O2 sensors as they already have the other connector attached - just disconnect the old one, remove old O2 sensor, install new sensor, reconnect cable. The hardest part is actually getting the old O2 sensor off in the first place - as by the time they need to be replaced, they are probably corrosion welded onto the threaded bung. Just hit it with PB Blaster or similar penetrating fluid, see if that will help, if stuck, drive around, let it cool - hit it with some spray again, repeat. Sometimes the heat cycling with the penetrating fluid and the process of attempting to loosen the sensor will free it. Exhaust system will burn the penetrating fluid off, so the car will smoke a bit, but should be too bad.

Depending on which O2 sensor it giving you problems, you'll have to pull back the carpet from the center console area or underneath the passenger front seat. In the cases where you get a consistent P0420 code and the catalytic converter is working (tail pipe sniffer test to verify) - then it is likely the downstream (post-cat) O2 sensor is faulty. The upstream (pre-cat) is the main O2 sensor that provides air/fuel feedback in closed loop mode. If you fuel mileage suddenly tanked, dropping 20%-30% or more from usual - then it is likely the upstream sensor is going. You are correct about the sensors - there are pretty "fragile" - shouldn't really be messing with the probe end at all, if you can. They usually slide on a piece of cardboard tubing to protect it, otherwise, just be careful. If you do accidentally touch it - no worries, just DON'T try and clean it, as that will definitely kill the O2 sensor. The working bit is behind that outer shield (cap with a bunch of little holes in it) - contact with the shield is really no big deal, just as long as contaminants don't get past and touch the probe inside.

For the MAF sensor - I'd try and clean first before replacing it. They make specail MAF cleaning sprays - CRC branded one is well liked. Almost any solvent that doesn't leave any residue behind can be used - just make sure the solvent is non-clorinated (will eat plastic right away) and dries completely before you reinstall the MAF. Don't touch the sensing wires of the MAF - just use the spray of the solvent to wash deposits off. May take several good blasts to clean off the wires. Don't forget to clean the little "bulb" next to the MAF - that is the IAT sensor, can get pretty gummy and nasty.

PCV valve replacement is also a good idea, if you haven't already done so. Cleaning the throttle body would also be high on my list as well - if anything, will atleast improve your idle quality.

Catalytic converter can be replaced - I know they do sell it as a bolt on section from the frontpipe to the resonator - but that is some serious $$$$. You can cut the old cat off and install a new one - some of the cats have stepped ends, so you can slide them over the existing piping and use clamps to sell them off. If you are looking a more permanent solution, welding in a new cat, a muffler shop should be able to do this quite quickly and fairly inexpensively.

The emissions policy makes absolutely no sense to me.

anyway all they did was test the system at the shop. $139... go back to test station, test failed and waived. no test for 2 years.

they said if I do any repairs after the waiver they will not count towards the next waiver. my estimate was a little over $1000.

this is the dumbest policy ever.

the cat, the part that un-pollutes, is the most expensive repair on the estimate. so in 2 years, theoretically, I can go and change one O2 sensor, get waived, 2 years after that get the next O2 sensor, get waived, and I'd never have to change the cat, I could just get an estimate every 2 years after that and get waived indefinitely and keep polluting the air.... this would be the best option for a poor person and if the purpose of the policy is for cleaner air, why wouldn't you just count all emissions repairs towards the next waiver. to a poor person, they can pay $400 and fix it, or they can pay just over $100 and fix nothing....

Anyway the test at the shop said:

no exhaust leaks found

cat bad

that'll be $139....

the repairs on the estimate were:

replace both O2 sensors - $200+ each with labor, I will really attempt to just do this myself, and I'll do this first.

clean MAF - it's $20 and I offered to pay them to just do it right there but they didn't seem all that interested in doing it... WTF? anyway I'll do this with the O2 sensors

replace cat - I think I'll go to our local pull n save and see if i can find a compatible corolla with low mileage and take the cat from that.... i am assuming i will need to bring a hammer maybe just a hacksaw and deal with the bolts later.... maybe snag the O2 sensors while I'm at it.

now,.. on the cat, I was told that only an oem replacement WILL work, one made for it will PROBABLY work, and a universal will PROBABLY NOT work as far as clearing the 420 code because the ECU is too specific. oem is way pricey, and the one made for it they are quoting me $389, and I got quoted universal at $240 total with labor.... I'm gonna still try and get an old low mileage one... providing the car was one that just got totaled and not one that was turbo'd and raced into the ground.

so, clean MAF and change front O2 sensor first, I was told this is for the engine's sake.

then the Cat and rear O2 sensor, I was told this doesn't matter to the car, just the air.

am i going about this right?

univ weld in 3500 pound car n under cat 80$

Depends on where you live at and the rules around replacement catalytic converters. Some places will automatically fail the car if it does not use OEM type cat (i.e., California), some don't care if it is OEM or universal.

As for replacing the O2 sensors, the rear O2 sensor (post-cat) only job is to report the P0420 code. Basically it compares the front O2 sensor signal to what it reads - if the signals look too similar, the ECM assumes the catalytic converter is bad. This also assumes that the post-cat O2 sensor (rear O2) is working properly. More times than not, that sensor is faulty and will trigger the P0420 CEL. Front O2 sensor, only replace if you noticed that your fuel mileage is dropping significantly. That is the primary sensor that is used for closed loop operation, if that sensor starts to go, first sign is generally a sudden drop in MPG.

MAF sensor is pretty easily to clean - worth a shot to clean, same goes with the throttle body and PCV. Cleaning those, replacing the PCV can greatly improve idle quality. Will also eliminate other potential culprits to help you diagnose this P0420. A bad engine coolant temperature sensor can cause a P0420 code to come on, exhaust leak is also pretty common for P0420 issues. Though, you have to keep in mind that it could very well be a bad catalytic converter - but I wouldn't take the shop's word on that yet.



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