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By mjatkins54, June 2, 2015

I have a 03 toyota corolla and have had the check engine light on for 2 years now the code that comes up is a p0420. I have replaced the cat 2 times I have replaced manifold gasket and donut gasket 2 times and I have replaced the O2 sensors 2 times front and back. Also I havereplace spark plugs and mass air flow sensors and the light is still on is it possible that I need to have my ecm/ecu reflashed

Parts that have been changed

catalytic converter 2 times

O2 SENSORS up stream and down stream 2 times first were with Bosch second time with Nippondenso

exhaust manifold gasket and donut gasket at the catalytic converter mass air flow sensor and spark plugs

When you replaced the O2 sensors were they charred? And the Cat, was it plugged up both times? How's your oil consumption?

It's not burning oil O2 sensors really didn't look bad I have no back pressure problems so I know the cat is still good and I haven't had the ECM reflashed it all and I hear a lot of people saying that Corollas need to have the ECM reflashed

When you replaced the O2 sensors were they charred? And the Cat, was it plugged up both times? How's your oil consumption?

That P0420 code can pop on for a number of reasons. Very rarely does it actually involve anything wrong with the catalytic converter or even the O2 sensors. Unfortunately, lots of techs only look up the code and replace parts instead of properly diagnosing the problem - that's usually why you see lots of posts of upset owners that have replaced a number of cats + O2 sensors and still have the code get set.

That said - if the car is running well otherwise, it very well be due to the initial threshold set in the ECM. An ECM reflash will help in that case (also the same fix for the excessive sulphur -dioxide issue with some 9th gen Corollas). Here is the catch - only the Denso ECM can be reflashed, the 9th gen Corollas with a Delco ECM cannot get flashed. Those cases, the ECM is just replaced.

Looks like you are also outside of the "Toyota specific major emissions components warranty" of 96 months / 80K miles. Reflash can be done with just the diagnostic charge - usually around $85-$95 - way less than a cat or O2 sensor replacement. If you are unlucky and have the Delco ECM - you'll be hit with a pretty steep bill for the ECM - street prices go anywhere from $1000 to $2000 easily. Some opted to get used ones from a salvage yard to save on costs, if you can find still find them.

Another option - trick the ECM. Look online for the sparkplug defouler mod on the downstream O2 sensor. Basically it moves the downstream O2 sensor just outside the bulk of the exhaust stream and "tricks" the ECM that the resulting waveform is within preset thresholds. Doesn't work for every single case, but due to the very low cost (couple of bucks) - usually the next thing some owners try if they are unsuccessful in getting the ECM flashed.

Since you've started a paper trail on the replaced O2 sensors and catalytic converters - might be able to show that to the dealership and have them consider you for a "good-faith" warranty work. Varies from dealership to dealership - some might cover 100% of the costs, even if the car is outside the warranty period - some might cover some portion of the repair. You won't know until you ask. Good luck.

How well does it run? Does code P0420 appear right away, or when it misfires at low speeds perhaps?

I have an 04 Corolla S, and I've had the check engine light on a good while. When tested, I also got the P0420. Oil consumption is low, engine runs well, fuel economy is great, with 155,000 miles. I've noticed, when the battery is unhooked, the CEL will go away for a while, then may come back in a few hundred miles later. I am not living in a state at the moment where I have to get an emissions certificate, but if I go back to Georgia, I suppose I will have to work to resolve this.

P0420 code is pretty common. Generally speaking, this is just an emissions related issue - so overall drive-ability will not be affected.

Code indicates catalyst efficiency is below threshold - can be cause by a number of things. Could be a dead or dying downstream O2 sensor (post-cat sensor), faulty engine coolant temperature sensor, leak in exhaust system, or could actually be a bad catalytic converter. OEM ones do have a limited lifespan - generally anything after 100K miles and you've done pretty well. But not uncommon for the OEM one to have 150K-200K+ miles before they actually go bad. A few have gone so far that the odometer actually stopped working (299,999) and are still going strong.

Since you mentioned the car is running well, good fuel economy, no excessive oil consumption - I'm leaning more toward a bad downstream O2 sensor or exhaust system leak. As there are no emissions requirement in your area - I'd just leave it. Won't hurt the car - just be sure to check the DTC from time to time - make sure it is not something else pops up.

Sorry to quote you, does the p0420 have an effect on fuel economy?

No change to driveability or fuel economy. The car will actually run like nothing happened. The EVAP system is only designed to control the emission of gasoline vapors from the car.

But if you live in a region where they do an I/M emissions test (smog test) - this would be an automatic failure.

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