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Toyota Corolla 2001 With Error Code P0420


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#1 ajay

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Posted 03 September 2006 - 08:10 AM

Hi there!

I have a Toyota corolla 2001 with 82,000 miles on it. I consider myself very conscious about the mileage I get. Here is my story. I was getting 29-31 mi/gallon mixed highway and city driving and I was kind of concerned. Then, I changed my original Denso spark plugs (iridium) with Bosch (Platinum) and my fuel economy went down to 23 mi/g. I heard from my friend that if you change the spark plugs then you need to reset the computer so that it can readjust the air-fuel ratio as per the plug design conditions. I didnít like the new plugs so, I cleaned the old plugs as they were looking ok and I got my mileage back to 29-30 (I had also reset the computer this time). I wasnít still happy with the mileage so I researched and cleaned MAF sensor with Electronic Part Cleaner. I drove about 400 miles without any problems. Then, on next gas filling about 60-70% of my driving was in the city (about 170), and after a trip of about 100 miles on freeway check engine light came on. I didnít check the code and disconnected the battery to get rid of engine lights. I drove like 500 miles without any problems (city as well as freeway) and then check engine light came on freeway driving. This time I checked the error codes with a scanner and it shows the following conditions:

Code P0420 : Catalyst system efficiency below threshold (Bank #1)
Speed 60 mi/hr
RPM 2906
Engine Load 30.6%
Temperature 189 (F I guess)
Fuel Pressure -5.5 -3.1 psi

Note: I have been using different fuel system cleaners in the past 2-3 months. I guess, I had used about 2-3 12Oz cleaners in this period. When I checked the code, that time I had used fuel economy improver (Gumout).

I researched a lot about this P0420 code and now I am very confused. The direct answer says Cat Converter (CC) may be bad but many people had check engine light come again even after replacing CC and my car is not that old as well. Could it be oxygen sensor? If yes, then which one?? When my friend scanned the code she checked the oxygen sensor and she said its working fine but she asked me to search the code first then we will see (Sheís not a professional). What is the time response for an O2 sensor??

I forgot to mention that, I didnít have any drivability problems except a bit slow acceleration (it could be just my mind as well). Today, I have changed my old spark plugs with new Denso IK-16 and following an internet reading, I drove the car with full A/C, two windows open and fans on at full speed to put extra load on engine. The idea was to heat the CC and get rid of any accumulation of contaminants. I could feel faster acceleration with little gas paddle pressed. I havenít reset the computer yet hoping that it will turn off by itself if CC and O2 sensors are good. Does it sound reasonable?? What else could be the problem??

I will really appreciate any input in this regard.

Thanks a lot, Ajay

#2 fishexpo101

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Posted 03 September 2006 - 08:16 PM

P0420 code refers to the downstream O2 sensor (the after the cat) reporting that the catalytic converter is not as efficient as it should be. On the 8th gen Corollas (1998-2002) - this is mostly due to a dirty MAF causing the system to think the cat is bad - sometimes it goes away sometimes it does not. In that case, might be a good idea to backprobe the rearmost sensor and see if it is acting correctly. Might be a bad heater on the sensor - have to check to be sure. This is one of those codes that doesn't impact drivability - just shows how over-sensitive the ECM threshold level is. I'd do what you suggest in the later part of your post - just drive it out. Loading up the engine with all the accessories is not neccessary - just punch the gas pedal to the floor. The extra gas dumped in there will actually heat the cat faster. Reset the ECM and see if it comes back. Fuel additives could also be causing some of these issues - I'd wait a few tanks of gas before I use more additives. Many tend to ruin sparkplugs or cause excessive deposits to form elsewhere, like the cat and O2 sensors.

#3 Bikeman982

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Posted 03 September 2006 - 08:22 PM

Hi there!

I have a Toyota corolla 2001 with 82,000 miles on it. I consider myself very conscious about the mileage I get. Here is my story. I was getting 29-31 mi/gallon mixed highway and city driving and I was kind of concerned. Then, I changed my original Denso spark plugs (iridium) with Bosch (Platinum) and my fuel economy went down to 23 mi/g. I heard from my friend that if you change the spark plugs then you need to reset the computer so that it can readjust the air-fuel ratio as per the plug design conditions. I didnít like the new plugs so, I cleaned the old plugs as they were looking ok and I got my mileage back to 29-30 (I had also reset the computer this time). I wasnít still happy with the mileage so I researched and cleaned MAF sensor with Electronic Part Cleaner. I drove about 400 miles without any problems. Then, on next gas filling about 60-70% of my driving was in the city (about 170), and after a trip of about 100 miles on freeway check engine light came on. I didnít check the code and disconnected the battery to get rid of engine lights. I drove like 500 miles without any problems (city as well as freeway) and then check engine light came on freeway driving. This time I checked the error codes with a scanner and it shows the following conditions:

Code P0420 : Catalyst system efficiency below threshold (Bank #1)
Speed 60 mi/hr
RPM 2906
Engine Load 30.6%
Temperature 189 (F I guess)
Fuel Pressure -5.5 -3.1 psi

Note: I have been using different fuel system cleaners in the past 2-3 months. I guess, I had used about 2-3 12Oz cleaners in this period. When I checked the code, that time I had used fuel economy improver (Gumout).

I researched a lot about this P0420 code and now I am very confused. The direct answer says Cat Converter (CC) may be bad but many people had check engine light come again even after replacing CC and my car is not that old as well. Could it be oxygen sensor? If yes, then which one?? When my friend scanned the code she checked the oxygen sensor and she said its working fine but she asked me to search the code first then we will see (Sheís not a professional). What is the time response for an O2 sensor??

I forgot to mention that, I didnít have any drivability problems except a bit slow acceleration (it could be just my mind as well). Today, I have changed my old spark plugs with new Denso IK-16 and following an internet reading, I drove the car with full A/C, two windows open and fans on at full speed to put extra load on engine. The idea was to heat the CC and get rid of any accumulation of contaminants. I could feel faster acceleration with little gas paddle pressed. I havenít reset the computer yet hoping that it will turn off by itself if CC and O2 sensors are good. Does it sound reasonable?? What else could be the problem??

I will really appreciate any input in this regard.

Thanks a lot, Ajay


There are a lot of people that have had the P0420 fault code problem. Some have changed the O2 sensors and some the catalytic converter (some have changed both, or all sensors and the c/c). Normally if the problem is fixed the CEL goes out, but the ECM may have to be reset (the fault cleared). I read that if you disconnect the battery, the diagnostic is reset (you may have to watch that you don't wipe out your radio code and settings).

Edited by Bikeman982, 04 September 2006 - 11:39 PM.

#4 friendly_jacek

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Posted 04 September 2006 - 11:19 PM

P0420:
After engine and catalyst are warmed up, and while vehicle is
driven within set vehicle and engine speed range, waveforms
of oxygen sensors have same amplitude (2 trip detection logic)

Trouble Area:
Gas leakage on exhaust system
Oxygen sensor
Heated oxygen sensor
ThreeĖway catalytic converter

The idea is to check for exhaust leak first then the front sensor and rear sensor. If everything checks out, cat needs to be replaced.

#5 Bikeman982

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Posted 04 September 2006 - 11:43 PM

P0420:
After engine and catalyst are warmed up, and while vehicle is
driven within set vehicle and engine speed range, waveforms
of oxygen sensors have same amplitude (2 trip detection logic)

Trouble Area:
Gas leakage on exhaust system
Oxygen sensor
Heated oxygen sensor
ThreeĖway catalytic converter

The idea is to check for exhaust leak first then the front sensor and rear sensor. If everything checks out, cat needs to be replaced.

Sounds like a logical plan of action for troubleshooting purposes.
Check for leaks, replace the cheaper parts before the expensive ones.
CEL should clear itself once system is operating within correct parameters.

Edited by Bikeman982, 04 September 2006 - 11:44 PM.

#6 thewizzard

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Posted 28 July 2008 - 02:30 AM

Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings to estimate how well the catalyst is actually doing its job of removing pollutants from the exhaust.

The upstream O2 sensor will typically show a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do much. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.

When the converter reaches about 600 degrees F, it is hot enough to start reacting with the gases in the exhaust. This is called the catalyst's "light off" temperature. The converter now starts to clean the exhaust and remove the pollutants. This causes a sudden drop in the switching activity of the downstream O2 sensor, and the sensor's output voltage levels off to an average reading of around 0.45 volts. This tells the OBD II system that the catalyst is doing its job and that everything is fine.

The OBD II catalyst monitor only runs under certain operating conditions, which typically require a combination of city and highway driving on the same drive cycle. If the catalyst monitor has run and no faults are found, the converter should be functioning properly and the vehicle should be in emissions compliance. But if the catalyst monitor runs and finds too much switching activity in the downstream O2 sensor after the converter is hot and the vehicle is being driven, it may set a P0420 code and turn on the Check Engine light.

Does this mean the converter has failed and that the vehicle is polluting? Not necessarily, but technically speaking the vehicle is NOT in emissions compliance if it has a code and the Check Engine light is on.

Most states now use a quick OBD II plug-in test to check emissions compliance on 1996 and newer vehicles. The plug-in test is faster, easier and cheaper to do than a loaded mode tailpipe emissions test on a road simulator or dynamometer. The rules in most states say to pass the OBD II plug-in test, the vehicle (1) must have a fully-functional OBD II system (Check Engine lamp works and the diagnostic connector communicated with the engine computer), that the Check Engine light must be off (not commanded on), and that there are no current codes in the computer's memory.

Consequently, if the Check Engine light is on and you have a P0420 code or any other code, you will FAIL the test -- unless the state allows you to take an alternate tailpipe emissions test to see if your vehicle is actually polluting. Some states allow this but others do not.

If your state allows an alternate tailpipe test -- and your car passes -- don't worry about the converter code. Your OBD II system may be over-reacting to a perceived emissions problem that does not yet exist.

Every vehicle manufacturer determines the cutpoints at which various codes are set as part of their emissions compliance testing that is required to achieve emissions compliance for a new vehicle. Some set their cutpoints a little conservatively to minimize the risk of an emissions recall. In other words, they would rather pull the emissions trigger sooner rather than later if a potential emissions problem is developing in a vehicle. Consequently, the converter may not be operating at peak efficiency but is still functioning well enough to pass an actual tailpipe emissions test. The exact point at which the light comes on may be when catalyst efficiency drops to 96%, 95%, 92% or what ever number the vehicle manufacturer's tests have shown it adversely affects emissions. The law requires the light to come on if emissions exceed the limit by 150%.

Why did the converter go bad? Under normal use, converters are engineered to last upwards of 150,000 miles. But any number of things can make it fail prematurely. The most common cause is contamination of the catalyst because the engine is burning oil or leaking coolant internally (leaky head gasket or a crack in a combustion chamber or cylinder). Converters can also be damaged if they overheat due to ignition misfiring that allows unburned fuel to pass through into the exhaust (check for a fouled spark plug or bad plug wire). The same thing can happen if the engine has a bad exhaust valve that leaks compression into the exhaust (check compression).

GETTING PAST THE TEST

Okay, back to the P0420 code and the Check Engine light. If you live in a state that does not allow an alternative tailpipe test if your vehicle fails an OBD II plug-in test, you're screwed. You have to fix the fault to turn out the light and pass the test. That means replacing the converter even if it is marginal or still functional.

The only way to accurately measure converter operating efficiency is to sample the exhaust gases ahead and behind the converter with an infrared exhaust analyzer. These are professional grade machines that cost from $2,400 for a basic hand-held 2-gas analyzer up to $4,400 or more for a 5-gas model. Operating an exhaust gas analyzer requires a good understanding of the various exhaust gas readings and what they mean, so it's not something that is within the ability or budget of a typical do-it-yourselfer. The only other alternative is to take your vehicle to a new car dealer or repair facility that has an exhaust analyzer and have them test your converter for you (which will likely cost $100 or so for the test).

You're probably wondering why you can't just clear the code just prior to taking the emissions test. First, disconnecting the battery on a late model vehicle with OBD II usually does NOT clear the code -- and can mess up a lot of calibration settings that can have an averse effect on driveability until the computer "relearns" the settings. And on some vehicles (like Chrysler Minivans and certain European luxury cars), disconnecting the battery can disable the climate control system and audio system in the vehicle.

What about using a plug-in scan tool or scanner software to clear the code and turn off the Check Engine light? This is the safe way to clear codes, but the catch is you're car will not be allowed to take an OBD II plug-in test until all of the required monitors have run -- which means you have to drive it until the catalyst monitor runs. If the problem is still there (which it likely will be), the code will reset and the Check Engine light will come back on. Now you're right back to square one.

So what's the fix? I hate to say it but you'll probably have to bite the bullet and replace the converter. Aftermarket converters are less expensive than OEM converters, and installing it yourself will save even more money. It's a difficult job because you have to get the car up off the ground (always use safety stands or ramps, not just a jack) and you're working on your back. What's more, the bolts that connect the converter to the exhaust will likely be rusted solid, so plan on a lot of sawing, pounding, chiseling and swearing. Also, make sure the new replacement converter is "OBD II compliant" and is the same type as your original converter. Don't waste your time trying to install a used converter because there's no way to know what condition it might be in.

Removing the converter and replacing it with a piece of straight pipe doesn't work either because the OBD II system will pick that up right away. So will a visual inspection under your car. You may also be fined for "tampering" if you're caught.

Worst case scenario: you replace the converter with a new one, drive the car and Check Engine light comes back on. The P0420 code has returned. Now what? This kind of situation can be prevented by doing some additional diagnostics and research BEFORE your replace the converter.

If the downstream O2 sensor is bad (heater circuit not working, loose or corroded wiring connector, contaminated sensor element, etc.), the OBD II system should detect the fault and set an oxygen sensor code. The same goes for a bad upstream O2 sensor. In either case, the presence of an O2 sensor code should prevent the catalyst monitor from running and setting a false P0420 code. Of course, this is ideally speaking and nothing is ever ideal. Sometimes a faulty O2 sensor is not bad enough to set an O2 sensor code but is off just enough to affect the accuracy of the catalyst monitor.

To minimize this risk, it's a good idea to check the operation of the O2 sensors with a scan tool or scanner software. You should see normal switching activity in both sensors shortly after the engine is started, with the O2 sensor voltage switching back and forth between rich (over 0.8 volts) and lean (less than 0.3 volts). A flat line O2 sensor reading or one that shows little switching activity is a bad sign. The downstream O2 sensor should slow down and go flat when the converter lights off (if the catalyst is working). If it keeps on switching like the upstream sensor, it tells you the catalyst is probably bad and the converter needs to be replaced.

You should also inspect the exhaust system and converter for leaks. "False" air can enter the exhaust through leaks and upset the O2 sensor readings, causing them to read leaner then normal.

Also, you need to do a little research. Check with a new car dealer or search online for any technical service bulletins that may relate to your problem. In some cases, a "reflash" of the engine computer may be required if the vehicle has a history of being overly sensitive with the catalyst monitor.

#7 erffuller

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Posted 03 August 2008 - 08:18 PM

Hi there!

I have a Toyota corolla 2001 with 82,000 miles on it. I consider myself very conscious about the mileage I get. Here is my story. I was getting 29-31 mi/gallon mixed highway and city driving and I was kind of concerned. Then, I changed my original Denso spark plugs (iridium) with Bosch (Platinum) and my fuel economy went down to 23 mi/g. I heard from my friend that if you change the spark plugs then you need to reset the computer so that it can readjust the air-fuel ratio as per the plug design conditions. I didn't like the new plugs so, I cleaned the old plugs as they were looking ok and I got my mileage back to 29-30 (I had also reset the computer this time). I wasn't still happy with the mileage so I researched and cleaned MAF sensor with Electronic Part Cleaner. I drove about 400 miles without any problems. Then, on next gas filling about 60-70% of my driving was in the city (about 170), and after a trip of about 100 miles on freeway check engine light came on. I didn't check the code and disconnected the battery to get rid of engine lights. I drove like 500 miles without any problems (city as well as freeway) and then check engine light came on freeway driving. This time I checked the error codes with a scanner and it shows the following conditions:

Code P0420 : Catalyst system efficiency below threshold (Bank #1)
Speed 60 mi/hr
RPM 2906
Engine Load 30.6%
Temperature 189 (F I guess)
Fuel Pressure -5.5 -3.1 psi

Note: I have been using different fuel system cleaners in the past 2-3 months. I guess, I had used about 2-3 12Oz cleaners in this period. When I checked the code, that time I had used fuel economy improver (Gumout).

I researched a lot about this P0420 code and now I am very confused. The direct answer says Cat Converter (CC) may be bad but many people had check engine light come again even after replacing CC and my car is not that old as well. Could it be oxygen sensor? If yes, then which one?? When my friend scanned the code she checked the oxygen sensor and she said its working fine but she asked me to search the code first then we will see (She's not a professional). What is the time response for an O2 sensor??

I forgot to mention that, I didn't have any drivability problems except a bit slow acceleration (it could be just my mind as well). Today, I have changed my old spark plugs with new Denso IK-16 and following an internet reading, I drove the car with full A/C, two windows open and fans on at full speed to put extra load on engine. The idea was to heat the CC and get rid of any accumulation of contaminants. I could feel faster acceleration with little gas paddle pressed. I haven't reset the computer yet hoping that it will turn off by itself if CC and O2 sensors are good. Does it sound reasonable?? What else could be the problem??

I will really appreciate any input in this regard.

Thanks a lot, Ajay


#8 erffuller

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Posted 03 August 2008 - 08:23 PM

I have 2001 CE and have been adding sea foam to the gas. The last time I did it about
30 min later the check engine light came on. Having just read this thread, I disconected
the battery for a while, and off went the light.

#9 Bikeman982

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Posted 04 August 2008 - 12:55 AM

I have 2001 CE and have been adding sea foam to the gas. The last time I did it about
30 min later the check engine light came on. Having just read this thread, I disconected
the battery for a while, and off went the light.

Drive it for a while and see if the CEL comes back on.
It might be the Seafoam affecting the sensors.

Edited by Bikeman982, 04 August 2008 - 12:56 AM.

#10 erffuller

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Posted 04 August 2008 - 08:57 PM

I have 2001 CE and have been adding sea foam to the gas. The last time I did it about
30 min later the check engine light came on. Having just read this thread, I disconected
the battery for a while, and off went the light.

Drive it for a while and see if the CEL comes back on.
It might be the Seafoam affecting the sensors.


I've put about 100 miles on it so far and the lights
still off. So far.

#11 warder

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Posted 08 March 2009 - 01:21 PM

Hi,

I have a P04020 error showing on my wife's 2005 Corolla.

Using an ODBII scan tool I can see this:

O2 Sensor 2, Bank 1 - 0.800V
O2 Sensor 1, Bank 1 - 3.274V , Eq. ratio: 0.998

As I've read that an O2 sensor should read between 0.1 and 1.1 V, I'm wondering if the issue is actually a failed O2 sensor. Does anyone have any thoughts on this?

I have a o'scope and can look at the signals on the sensor, if I knew where it was located. Is this after the cat on the underside?

Thanks,

RW

#12 fishexpo101

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Posted 09 March 2009 - 10:44 AM

On the 2005 - you have two sensors - one is infront of the catalytic converter and the other right behind the converter.

As for the voltage ranges on your particular model year - the upstream sensor (pre-cat) should read between ~0V - ~5V, the downstream (post-cat) should read between ~0V - ~1V. The reason for the difference, is your upstream sensor is not an O2 sensor, it is an Air/Fuel sensor - very different beast. It acts more like a wideband O2 sensor than the typical O2 sensor (90% are narrow band). Most people get into trouble, because some parts exchanges (cross-references) have the 2005 Corolla listed having a standard upstream O2 sensor instead of the newer Air/fuel variant - this will immediately set a CEL. You can buy the right ones aftermarket or at the dealership - but they would be considerably more expensive than the older versions.

For the P0420 CEL - you will be looking at the downstream sensor - the one after the car. You should be able to tap the wiring by pulling back the carpeting under the passenger seat. I can't remember if you can get at the rear O2 sensor without pulling any up or not. Might have to remove the chair and center console to get more room - remember to disconnect the negative terminal on the battery first, most of the 2005+ Corollas I've seen had side airbags equipped - don't want to risk popping the bag accidentally as you pull the chair out. Backprobe the sensor from the wiring harness there - so that you can monitor it on the scope while your inside the car - not underneath the car, next to the hot exhaust.

Edited by fishexpo101, 09 March 2009 - 10:44 AM.