2001 Corolla Check Engine P0420 - Toyota Corolla, Chevy Prizm (1998-2008)

Hello everyone,

I am new to this message board and really hope someone can help me. My wife has a 2001 Corolla S. It's a 4 cyl with automatic transmission and air conditioner (not sure if that matters but more info is better than less). To be honest I am not much of a car guy. I change my own oil, belts and simple stuff like that. Her car's check engine light will turn on every couple weeks with the P0420 (Catalyst Efficiency Below Threshold - Bank 1) error. The car seems to run normally when the light is on but it seems to get worse fuel mileage. Where is the sensor that is giving the error? I thought that maybe I could clean it. I am an electronics engineer but like I said, no mechanic. If someone can give me an idea of where to locate the sensor, I would greatly appreciate it! If money wasn't so tight I'd go buy the service manual.

Do I need a special wrench to remove the sensor? Is there anything I should know before doing this?

Thank you very much for your help!!!

Eric

P.S. If this is a common error and someone knows the solution, please let me know.

That type of mileage is on the car? If over 80K miles, a bad O2 sensor might also be a culprit, that you might want to put up on your radar. Does the car consume a considerable amount of engine oil? If it does, how much? If oil consumption is excessive (a quart every few hundreds of miles) - it is possible that the catalytic converter is clogged or even poisoned (additives in the oil).

The P0420 code is pretty ubiquitous for this generation of Corolla. In most of the cases, the cause for the P0420 is just a tuneup issue or the ECM crying "wolf". First thing I would do would be to reset the CEL. That can be done by a handheld scanner and clearing the trouble code, or disconnecting the negative battery terminal for a minute or so to kill power to the ECM. Note that disconnecting the battery would make some systems very unhappy - like security systems on the car.

Taking it to a shop - I almost guarantee that they will say that the catalytic converter is bad and needs to be replaced and/or O2 sensors (pre-cat and post-cat) will need to be replaced. The problem is, even after you have those replaced, many times the CEL will return.

After the code is reset, see if it comes back. 8/10 times - it won't be back. If it one of those few that does come back - look to getting the car tuned up. Check the condition of the sparkplugs, check for vacuum leaks, clean the throttle body, clean the MAF sensor, etc. Do a search on P0420 and you'll see there are quite a few topics on it and potential solutions.

Her car has roughly 82K miles on it. It doesn't use any oil though. I have cleared the check engine light something like 10 times in the last 2 years and it is always the P0420 code. I was told it is most likely just crying wolf but I'm afraid that someday a serious engine light will pop up and be ignored. I don't trust our toyota shop as they have ripped me off multiple times (they stripped out the oil pan plug (60K tuneup, then claimed it was like that when it arrived even though they were the previous place to change the oil, etc.) That's why I change our oil now and know that it isn't using any oil to speak of.

I will take a look at the stuff you mentioned but like I said, I'm not a mechanic. I think I found the sensor on the Cat. I only see one sensor but I seem to remember reading somewhere that this year's model only has the one and the next year's model had 2. I was going to pull the sensor and give it a good cleaning with some electronics contact cleaner since it dries fast and doesn't leave anything behind.

The last thing I need right now is an expensive repair. Christmas is already about to be canceled, that would definately cancel it for this year.

Is the MAF sensor on the air intake before the air filter?

Thanks again,

Eric

That type of mileage is on the car? If over 80K miles, a bad O2 sensor might also be a culprit, that you might want to put up on your radar. Does the car consume a considerable amount of engine oil? If it does, how much? If oil consumption is excessive (a quart every few hundreds of miles) - it is possible that the catalytic converter is clogged or even poisoned (additives in the oil).

The P0420 code is pretty ubiquitous for this generation of Corolla. In most of the cases, the cause for the P0420 is just a tuneup issue or the ECM crying "wolf". First thing I would do would be to reset the CEL. That can be done by a handheld scanner and clearing the trouble code, or disconnecting the negative battery terminal for a minute or so to kill power to the ECM. Note that disconnecting the battery would make some systems very unhappy - like security systems on the car.
Taking it to a shop - I almost guarantee that they will say that the catalytic converter is bad and needs to be replaced and/or O2 sensors (pre-cat and post-cat) will need to be replaced. The problem is, even after you have those replaced, many times the CEL will return.
After the code is reset, see if it comes back. 8/10 times - it won't be back. If it one of those few that does come back - look to getting the car tuned up. Check the condition of the sparkplugs, check for vacuum leaks, clean the throttle body, clean the MAF sensor, etc. Do a search on P0420 and you'll see there are quite a few topics on it and potential solutions.

you can drive the car with that code until you can afford to repair it, its strictly an emissions code.

The car has two O2 sensors - one is close to the catalytic converter and one is further up on the exhaust down pipe, almost right after the exhaust manifold. I would not attempt to "clean" the O2 sensors, even with a solvent that evaporate quickly, it generally will dissolve whatever is around the O2 sensor, almost guaranteed that will kill the sensor.

Bitter is right about being an emissions related code. If your area does mandatory smog testing, any CEL is grounds for an automatic fail for emissions testing, even though the actual tailpipe emissions are within spec.

Could be something as simple as an exhaust leak close to an O2 sensor that is causing the CEL to pop up. The exhaust donuts are known to almost completely disintegrate - if there was excessive clearance there, it could cause the ECM to pop that code. Sensors are the next likely culprit - they have to be backprobed and have their signal waveforms verified. Many times, shops just blindly change sensors without actually checking them. Sensors could be fine, could be a wiring issue or chassis electrical noise issue. If everything else has been checked out, there is a possibility that the catalytic converter is bad. Depending on where you live at - it could be under the extended emissions warranty (8 years/ 80K miles) - if it is truely a bad cat, might be able to get some help from Toyota, but you won't know until you ask.

As for future problems down the road - hard to say, as there are many possible culprits for a P0420 code. Some have driven around with that code for years and not had any problems. Others have driven with that on for a few months, ended up having to rebuild the engine. Since it seems to the former type in your case (CEL ~10x over the past two years) - you probably can let it go a little longer. But don't let it get too far away from you, be nice to have that P0420 go away permanently.

I live in California and we have to get smagged every 2 years. I have been getting this error, and clearing it, since before the last smag was due. I cleared the error and it passed fine so at least it isn't a big problem. I just really hate having the light come on. It always seems to turn on when we are on a trip. It will turn on everytime we drive to Vegas. Other than a trip to vegas twice a year, we do not drive much so it makes sense that it would turn on when we are actually driving farther. My biggest worry is that sometime that check engine light is going to pop on, and we are going to ignore it, and it be something that causes engine failure.

It seems that a lot of these cars have this error and you would think Toyota would fix it with a recall. About a year ago I cleaned the sensor on the air intake. You are going to cringe but I actually WASHED it with pure water and the contact cleaner. I thoroughly dried it out and kept air blowing through the sensor for a couple days before hooking it back up. I looked at it first to see if there was anything stuck in there and didn't see anything but the cleaning did seem to delay the check engine light turning on for a while. I'm sure that isn't in any instruction manual, but I have a background in electronics and knew that as long as power wasn't supplied to the circuit, and it was completely dry before use, it should be ok.

We are the original owners. I don't live in a dusty area and I don't do fuel or oil additives other than the STP injector cleaner that I've used twice.

Last thing. This car has NEVER gotten near the mileage that the sticker said (39 highway/33 city). The best mileage we've ever had was 31 MPG (driving to Vegas - all freeway) but usually it is around 29h/28c. I don't know if this has anything to do with the problem but I thought I might as well say something about it. It just seems that this car has always had a little bit of a lemon scent to it. Other than the mileage and P0420 CEL, it has been an ok car.

I will check through the list of things you've said to look at (if I can figure them out) and see if anything fixes the problem. I still would appreciate any further comments. I'll also post my findings and anything that solves the problem. With how long it takes for the CEL to come on after erasing it, I might think something is fixed and then a month later have the CEL pop back on. I'll be sure to post that too. Hopefully it is a cheap fix. I would much rather cell this car and get something that we both like better, but not until the economy rises from the grave!

Thanks for your help!

Eric

For P0420 to be set, the ECM compares the signal waveforms of the heated oxygen sensors upstream and downstream of the catalytic converter (pre and post cat O2 sensors) to determine whether or not catalyst performance has deteriorated, since the ECU has no way to actually check the catalytic converter aside from reading what goes in and what comes out. This assumes that the sensors are working perfectly, there is no exhaust leak, and the ECM is not confused.

After the engine and catalyst are warmed up to operating temps, and while the vehicle is driven within a preset engine speed RPM, and the resulting waveforms of the upstream and downstream O2 sensors have the same amplitude and waveform, and 2 trip detection logic (has to happen twice, or it is thrown out as a electrical glitch). Only then will the P0420 CEL can be set.

This CEL just indicates that the readings from the upstream and downstream O2 sensors look too similar and because of that, it "assumes" the catalytic converter is faulty. Problem is, in many cases - the O2 sensors and catalytic converter are perfectly fine. Have to trace down the part that is popping up the CEL, if those are exhausted - then you have to look at the O2 sensors and finally the catalytic converter.

Since you noted that the fuel economy is not as high as you would like (depends on the transaxle on the car - 3-speed auto/4-speed auto/5-speed manual) and you get a P0420 code. It would lead me to believe that the engine coolant temperature sensor is faulty so that the car will not warm up normally (hurts fuel economy), plugs or injectors are fouled (hurts fuel economy), or the O2 sensor has gotten lazy - i.e., didn't quite fail, but not reacting to the air-fuel mix as it should (hurts fuel economy). Since the CEL pops up as often as it does, I would suggest getting an OBD-II code reader to verify the codes. If you can get one with data-logging capability, all the better, as it would be nice to see exactly what the sensors are outputting.

Then again, the lowish fuel economy numbers may be completely unrelated to the P0420 code.

Also possibly that the car isn't driven enough between CEL events. Hard to say without being right there with the car. You've hit one potential culprit for poor fuel economy, the MAF sensor. Doubt the water hurt it, as this is exposed to some moisture from the incoming air. Completing a comprehensive tuneup (plugs, clean throttle body, fluid exchanges, coolant drain and refill, etc.) may help with overall fuel economy. Tire air pressures, suspension, steering, and alignment are other things to look into to see if you can increase fuel economy.

I feel your frustration. Just for comparison, my 2002 Corolla (4-speed auto) has over 150K miles, still on original sensors, doesn't consume any oil, and gets between 36-38MPG on highway on my daily commutes (500 mile/week). Generally see ~30MPG low if city driving, as high as mid 40's on steady state cruising. But I get that dreaded EVAP code seemingly every other year - right when I have to smog the car. Here, resetting the ECM will automatically fail my smog, since they are querying the I/M status on the ECM. If I clear the ECM of codes, it also resets the I/M readiness monitor. Usually fuzzing with the vacuum hoses, and whatnot by the EVAP components usually does the trick to let me pass. But it kills me that the CEL stays off for 24 months, only to pop back on when I get ready to smog. This year, the CEL popped on while I was waiting in line to get smogged.

Ouch about the CEL lighting while waiting for the test!!! California is really strict about air quality, and it's a good thing. LA isn't as smoggy as is was back in the 80s and 90s. It used to look like a cloud was over LA all the time. It's less now and the strict smog rules have a lot to do with it.

My wifes car has NEVER, even when brand spanking new, gotten better than 32MPG. We originally purchased this car because for the gas mileage. Before we had a SUV that drank gas.

I appreciate all the time and info you've given me! I don't really know how much of the stuff you mentioned was inspected/replaced during the major service but the CEL happened both before and after that service was done. The plugs at least should have been replaced. I'll try to get some free time this weekend to pull the plugs and see how they look. With the MAF sensor, it did seem to help delay the CEL but it might just be coincidence. I've thought about replacing the sensor but I don't like throwing money away if it isn't the culprit.

If I pull the O2 sensors, is there an o-ring, gasket, or something that I will have to replace to get it installed correctly or is it just something I can pull, look at, and reinstall? I wouldn't think there would be anything but I want to be sure before doing something stupid! I figure it would be pretty easy to pull them, and see if there is anything visibly wrong. I could even take some pictures and send them to a friend that knows what they are doing when it comes to auto repair.

I do have an OBD II scan tool but nothing with data loggin or anything like that. I used it to find out the P0420 and to reset the CEL to off. It isn't the best OBD II scanner. I got it from Harbor Freight (item 94169) for around $50 or so.

Of course the CEL didn't start giving us problems until the warranty was expired. I think they had a relay hooked onto the CEL that kept it turned off until the mileage was over 36000. After that, the relay allowed power to be applied to the CEL.. hahaha

Anyway, thanks again for the explination and time! I really appreciate it and will look into what I can of the items you specified. I don't have the tools for some of the stuff so I will try what I can and see if it works. If not, then I guess I get to see the pretty light until I can afford to fix it, or sell it.

Thanks again,

Eric

Only thing I'd worry about pulling the O2 sensors, is that the wiring could get damage in the removal process or you could strip the threads on the sensor body. In either case, you would be forced to by a new sensor to replace it. I'd only do this if you are 100% sure the O2 sensor is dead. There really isn't anything to check on the sensor itself to check or adjust - aside from checking the connectors (inside the car, passenger side floor and under the seat, have to peel back the carpeting. Visually, a bad O2 sensor will look like a good one. I would definitely look for exhaust leaks, though - especially close to the cat and O2 sensors themselves.

If you have a multimeter handy, I would measure the voltages coming off the O2 sensor (backprobe the connector inside the car) - depending on the resolution and sampling rate of the multimeter - you might be able to see the voltage cyclical behavior, as the ECM shifts the air-fuel mix lean to rich and back. Or scanner with logging or freeze frame ability can read them directly from the ECM. Not as surefire as reading from the sensor direct, as the ECM itself could be interpreting the signals incorrectly from the O2 sensor.

LOL! at the relay on the CEL. Yeah, mine was flawless (no CEL) until about 32K-34K miles rolled on the clock. Which happened about a few months before I was scheduled to smog the car. Since then, did it popped up every other year - regardless of the miles, how it was driven, the ambient temps - almost exactly a week or two before I plan on smogging the car. Almost like it was following a calender on when to pop the CEL.

friendly_jacek

I will chime in about MPG. The previous (pre 2006?) EPA MPG figures were over exaggerated and few people got the EPA figures or more (hypermiling techniques). I have 2003 with the same engine and 4-speed auto and get 27-28 city and 30-32 hwy. I rarely got more, up to 36 mph, but only with low speed driving like 60 mph. If you drive over 70 mph or in congested LA hwys, forget about good MPG.

I guess you could get a cheap OBD2 interface on ebay that connects to a laptop or palm pilot. Then you could see the O2 sensors voltage graphs and that would show you if they work properly.

Good luck.

Is the MAF sensor on the air intake before the air filter?

The MAF sensor is on the lee side of the filter (so that the filter itself protects the MAF from debris).

Life got in the way and I didn't get to look at the car this past weekend. I appreciate all the info and will hopefully get some free time this weekend to poke around.

We owned 2 different Corollas and I can't remember if it was this one or the older one that had a recall about the exhaust manifold cracking and leaking exhaust. I remember toyota replacing the manifold but wonder, if it was this car, if the repair didn't entirely fix the problem. It was an actual recall so I'll have to go look it up.

I wish our local Toyota wasnt run by a bunch of crooks, then I'd just bring it in and let them figure it out. Our toyota would probably tell me I need a whole new engine.

Eric

Guest Billy D

I've had that code on my '01 LE and I got the Cat replaced-no more problems. I DID have a few other things checked first (O2 and MAF also replaced, MAF helped with mileage, but not 420 code).

The biggest reason I'd say your car's mileage is crap when the light comes on is because when the sensor is reading a bad Cat, bad data is sent to the engine, your mixture becomes richer (more fuel than necessary), and the mileage drops, as well as compounding any issues with a bad cat.

Also, this gen has had problems with the engines eating oil-usually claimed to be caused by pistons that are a tad small. Could be caused by oil disappearing through the exhaust system and clogging up the Cat. I don't have emissions testing here in KY, so to me it's not a big deal.

As for fuel economy, the best I've ever gotten is 41, and I was drafting nearly the entire tank. In city I average about 29-31, that's when I'm easy on the pedal.

One thing to try would be to get some carburetor/O2 cleaner and spray off the MAF/O2 sensors. I've done this and it helps with power and mileage. If you clean off both O2 sensors it's possible it could clear that code (if it's just a faulty sensor due to being dirty).

54 posts

I get a P0420, 441 and 446...

I also have an engine cooling fan that is ALWAYS on.

The temperature sensor is overpriced at $45, but it looks like I can kill 2 birds with 1 stone if I change mine...

Correct logic?

The 441 and 446 are obvious... my purge solenoid is *JUNK*... I pulled it and checked it out... yeah. It's junk. That's $120 I'm not going to spend any time soon.

Possible - at least that is likely to fix the fan running issue. If this is coming from an autoparts store - try and see if they'll let you try it out and bring it back for a refund or store credit if that is not the culprit. P0441/P0446 is almost alway the purge solenoid or CCV - though sometimes can come from any number of vacuum hoses and tank vent lines. P0420 is usually set with these others - though seems to be a different main culprit or culprits that cause this to pop up. Doesn't happen nearly as much on the 9th gen Corolla/Matrix/Vibe - since they relocated the EVAP components.

54 posts

Doesn't happen nearly as much on the 9th gen Corolla/Matrix/Vibe - since they relocated the EVAP components.

Yeah, definitely. When I saw where they hid the charcoal canister and purge solenoid, I was having severe difficulties trying to understand the logic of the engineers who decided to put them there.. The vac lines are all ok... but like I said, that purge solenoid is definitely bad. I applied 12v to it and it did not operate. So $120 will get rid of TWO of my codes...

Leaves the 420 though... I think I'll try picking up a temp sender and see what happens. Chances are it'll fix the fan problem at least, probably get me a couple MPG, too.

I would definitely look for exhaust leaks, though - especially close to the cat and O2 sensors themselves.

How exactly does one go about looking for exhaust leaks? Could you kindly tell me more?

Do you physically feel for exhaust gases leaking out? Or would you use your ears and try to find extra sources of sound along the exhaust tubing? Or would you spray soapy water over the exhaust pipes and look for bubbles?

I would definitely look for exhaust leaks, though - especially close to the cat and O2 sensors themselves.
How exactly does one go about looking for exhaust leaks? Could you kindly tell me more?
Do you physically feel for exhaust gases leaking out? Or would you use your ears and try to find extra sources of sound along the exhaust tubing? Or would you spray soapy water over the exhaust pipes and look for bubbles?

Foolproof method is to find a shop with a smoke machine. I realize you were probably interested in DIY, but it may be better in this case to shell out the $.