2004 Corolla P0420 Code - Toyota Nation Forum : Toyota Car and Truck Forums
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#1 Old 12-05-2010, 06:57 PM
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2004 Corolla P0420 Code

I live in Florida and my daughter lives in North Carolina. I am helping her isolate her problem via the telephone and e-mail. My daughter's Corolla has more than 100k miles. It's VIN number is greater than the cutoff number in TSB EG028-04 also known as the P0420 TSB. I have advised her to have an exhaust system visual check, check the plugs and wires, replace the PCV and the Coolant Temperature Sensor plus have the codes cleared and drive it a while to see if the P0420 code returns.

If the P0420 code returns have someone in her area check the O2 sensors for proper operation. If no one can check the O2 sensors, my gut instinct is to replace the upstream sensor as the next least expensive step. If the upstream sensor is bad(readings not fluctuating) which means both sensors are possibly generating stable, similar readings and the ECM should throw a P0420 code. If after replacing the upstream sensor she still gets a P0420 code that means the cat is bad because both sensors would be generating fluctuating, similar readings and the cat needs to be replaced. Are my assumptions correct? All comments will be greatly appreciated.
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#2 Old 12-05-2010, 09:19 PM
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Welcome, it seems a bit premature to replace the coolant temp sensor. If the plugs are original, they can be change to iridium NGK or Denso plugs. Clearing the code is the first step with this code, which is correct on your part. Probably wouldn't hurt to change the PCV valve, but that won't do much for the code.

Just an FYI, if you want to change the sensors/plugs/valves, stick with OEM, which is Denso or NGK, that is very important. I believe advance auto carries NGK sensors, which will be much cheaper than the dealer. VIN must be given when ordering PCV valve.

If the code comes back on, and no one can back probe the sensors, I would replace the downstream sensor first (under the car), as it can set off a p04020 more often than the upstream sensor (under the hood). This assumes that there are no leaks. If the light comes back on, and still no one can test the upstream sensor, that will need to be replaced and then the cat ($$$) if nothing else works.

Does she burn any oil? Does she have the sock exhaust? Does she use any oil additives? Did her mileage drop?
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#3 Old 12-06-2010, 06:31 AM
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Thanks for your reply, much appreciated, She did not say anything about burning oil and I don't think she would know about the exhaust. I did ask her about oil and gas additives and she said there hasn't been any used by her, but does not know what has been used during maintenance of the car. She did not mention gas mileage affects. I will ask about exhaust type and mileage.

If I don't get additional information this is the process I will recommend at this time: visual check of the exhaust system for leaks and repair if leaks found, change the original plugs, change the downstream O2 sensor, reset the codes and drive it. If the P0420 code is set again change the upstream O2 sensor, reset the codes and drive it. If the P0420 code is set again change the cat converter reset the codes and drive it. It the dastardly P0420 code returns we are all out of ideas and she will have to request a waiver from North Carolina emissions.
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#4 Old 12-09-2010, 12:19 PM
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I have a 2003 Corolla and have also dealt with the P0420 code problem.

Here's what I've discovered. I'll try to minimize the technical aspects.

Some of this you are already familiar with, but I will post it for the benefit of other readers.

The P0420 codes indicates that engine computer(ECM) has determined that the catalytic converter is not cleaning the exhaust as well as it should. This determination is done by monitoring the oxygen sensor readings (there are two of them on your exhaust system, one before and one after the converter). When the converter is working properly, the sensor readings will differ because the exhaust has been cleaned by the coverter in between the two sensors. If the readings are too similar to each other, the ECM assumes that the converter isn't cleaning the exhaust and the ECM sets off the P0420 code.

For most cars, this means the cat converter needs replacement. However, on the Corolla, that's not necessarily the case.

If you are lucky, you may get away with having your ECM reprogrammed.

Turns out that Toyota set the 'prescribed range' for the minimum difference in the sensor readings on the 2003 and 2004 Corolla very narrow. More narrow than it needed to be set to meet EPA guidelines. Thus, even though the ECM triggers the P0420 code, your emissions may still be within EPA limits. Toyota recognized this and issued a service bulletin, EG028-04 (You can google that bulletin number and find a copy of the bulletin).

The remedy in the TSB bulletin is to reprogram the ECM and put in a wider range for the oxygen sensor readings. The cost to reprogram (or 'flash' )the ECM is about $150.

Oxygen Sensors

Unless you are getting more MIL codes than just the P0420 code, I think its unlikely the problem is caused by a faulty oxygen sensor. Usually, a bad oxygen sensor will usually trigger it's own, separate MIL code in addition to the P0420 code.

However, since your car has over 100K, the threshold for replacement suggested by the sensor manufacturers, it probably makes sense to replace both sensors and see if that fixes the problem.

A faulty upstream sensor will usually cause the fuel mixture to become too rich resulting in high gas consumption and emissions. So if you've seen a reduction in your gas mileage the upstream sensor may be the culprit.

Replacement sensors will cost about $75-100 each plus labor to install the sensors. It's a fairly easy hour or so repair on the Corolla. I recommend getting the special socket for removing the sensor. It's a good $10-15 investment.

Replacing the Converter

For my car, the reprogramming didn't work. The car has 175,000 miles and the converter was just worn out. If your cat. converter needs replacement, the dealer will want over $1200 just for the parts. Probably close to $1500 with labor.

That was way more than I cared, or had to spend. I installed a good quality direct fit aftermarket converter which cost about $400 from an online parts supplier and carried a 5 year 50K mile warranty. I installed it myself but I estimate a garage would charge you $100 or less for labor to install it. I estimate the job should take an hour or so if you have the car up on a lift.

Doing it yourself on your back, under the car, will take much longer. The big unknown in the repair is whether the four bolts (two at the exhaust manifold and two where the pipe connects to the muffler) will release or break. I soaked mine in penetrating oil before trying to remove them. Be sure to use a six point rather than 12 point socket for better torque. It makes a world of difference on rusted bolts. My biggest problem was getting the old flange gasket off the exhaust manifold. I ended up using a cutting disk in a dremel to get it off.

I highly recommend buying the direct fit replacement kit that includes the resonator pipe as well as the catalytic converter. Also be sure to get the two replacement flange gaskets and replacement spring/bolts kits for where the assembly attaches to the exhaust manifold and your muffler. Don't try to reuse your old gaskets and bolts.

Install the entire converter/resonator assembly in one piece. Be sure to leave the joint connecting the covertor and resonator pieces loose until you have the assembly installed and connected to both the exhaust manifold and muffler. I found it helpful to attached the assembly to the hanging bracket in the center of the car before trying to reattach the front to the exhaust manifold. Once it's all in place, then tighten the u-bolt clamp around the joint holding the two pieces of the assmbly.

Even though your current resonator might be ok, getting the kit insures a better fit and make the replacement much easier. Don't go cheap with a generic converter. You'll regret it unless you plan to sell the car ASAP.

Last edited by aspen007; 12-09-2010 at 12:27 PM. Reason: typo
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#5 Old 12-09-2010, 02:46 PM
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Thanks for your insight and info. I have found Denso sensors and spark plugs for a reasonable price at Advance Auto Parts. I have also found at the same outlet Walker Direct-Fit Catalytic Converter and Resonator for a very reasonable price compared to dealer prices. I recommended my daughter replace spark plugs and have both sensors tested, replace whichever sensors are bad and reset the codes. If the P0420 code returns install the Catalytic Converter/Resonator from Advance Auto.

Walker Direct-Fit Catalytic Converter Part No. 53717 $262.59
Walker Resonator Assembly - Import - PC Part No. 55349 $58.99

I asked her to let me know what actions are taken and the results. I will post the results here for the benefit of others. P0420 is a difficult code to deal with.
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#6 Old 12-09-2010, 03:00 PM
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Those are the same Walker cat. converter and resonator parts I installed. They fit great and fixed the problem.

The only problem I had was that the Walker flange gasket (#31531 )listed for the front exhaust manifold wasn't the right size. The external diameter was ok but the diameter of the hole was larger than the OEM gasket. The rear flange gasket (31378) fit fine.

Good luck.
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#7 Old 12-11-2010, 04:54 PM
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Good News! Today my daughter's 2004 Toyota Corolla passed the North Carolina emissions test with flying colors - NO CODES. The P0420 has disappeared for now. Replaced spark plugs and downstream O2 sensor with Denso parts, reset the codes and drove the vehicle 110 miles prior to the emissions test.

As Mr. Spock would say - "It is not logical." We will gladly accept the results!
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#8 Old 12-05-2011, 11:31 PM
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Thumbs up My 2004 Corolla P0420 issue

During August 2010 my engine light came on while driving from Lake Powell back home to Salt Lake City, UT. The first mechanic I saw in the middle of nowhere told me that it was probably the gas I bought in AZ. He said that he sees a lot of Toyotas come through with their engine light on. He reset it and I drove home.

Sometime after getting back home the light came back on so I took it in to my mechanic whom I have learned to trust. He pulled up the P0420 code. He reset it and had me drive some more to see what happened. It came on again so I decided to just forget about it. November came and I needed to register my car and get the emissions done on it so I took it in and it failed because of the P0420 code. They told me to take it to a mechanic and get it addressed. So I returned to my mechanic and he suggested I get a new catalytic converter which I did. He ordered an after market cat to save me the $800. I paid $480 parts and labor. I then went to the emissions place and got the car passed and registered the car. I was happy.

Soon thereafter the engine light came back on. I couldn't believe it. I took it back to the mechanic and he reset the light and I left. Yep, it came on again. He then told me I had to take it to the dealership to get the computer "Flashed" and that only they could do it. I called the dealership in town and they said it would cost $90 to flash the computer. Being that the car was already registered (somehow passing the emissions) I let it go and drove an entire year with the engine light on.

Well, November 2011 came so I figured I'd call up the dealership and head on down, pay the $90 and get the car registered. The Toyota dealership I went to didn't know what I was talking about. I told them I had put a new catalytic converter in a year ago and that every mechanic I talked to said I need to have them to wave their magic dealership wand over the computer so I could be on my way. They claimed to not knowing what I was talking about and said they would verify that my computer has the most recent update for the catalytic converter. After an hour of waiting they said it did and that there wasn't anything they could do. They told me since I put an after market cat in it that the computer wasn't jiving with it well. They even suggested that it could have gone out already and tried to sell me a $1,200 Toyota cat. I left staring at my engine light again. I drove straight to my mechanic and they were dumfounded. The thought the dealership was crazy.

That night I decided to check out what the forums had to say and found this thread. I printed off the Toyota Technical Service bulletin that was posted and took it to a different Toyota dealership in Draper, UT (Karl Malone Toyota). They said they new exactly what to do. They "Flashed" it and I left drove off with the engine light off. All I paid was $60 after taxes.
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#9 Old 12-06-2011, 02:43 PM
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Just last month I replaced a cat on an '03 for the P0420. O2 sensors appeared to be reading correctly and the car had 165k on it.

The cat is part of the front pipe assembly right behind the front flange. I got a close look at it and shouldn't have a problem welding on a universal converter when mine goes out. I can get a universal for under $100 and am willing to try that before spending $1000+ on a toyota assembly.

JWD I have a feeling that you got lucky. The cat is probably "borderline" right now. It's cool that you got it through emissions so there's no rush to fix it if it comes back on but you and I both know that it will.

Look at your emissions printout (if you got one). Does it list your monitors? Was your CATALYST monitor set to READY? I wouldn't be surprised if it isn't ready yet which is why the light hasn't come back on yet. Here in Missouri you can pass the test with one monitor not set to ready (unless you previously failed for a cat code in which the catalyst monitor has to be ready before it will pass)
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