Catalytic Converter Code - P0420, But Light Went Out - Corolland: the Toyota Corolla Forums

I have an 03 Corolla LE with 177,000 miles. Fabulous car.

A week and a half ago, my check engine light came on (first time ever). Took it to an independent shop near me. Turned out to be P0420, catalytic converter. I decided to think things over, so I did not have the shop change anything. On the inspection repair order, the tech had written "DTC P0420 - catalytic converter efficiency below threshold Bank 1."

I live in L.A. and had just passed a smog check back in April of this year, so it seemed odd that the cat was suddenly failing. Unfortunately, smog tests are now done pass/fail, so you can't see the test numbers like before. And it didn't occur to me to ask for the actual numbers, which I will do next time.

About five days later, before I had a chance to get a second opinion from the dealer, where I usually go for service, the check engine light went out. It's still out and the car runs fine (and has run fine all along).

I found another Corolland post from June 2015 from a guy with an 03 LE just like mine with a P0420 code problem. https://www.corolland.com/forums/index.php?/topic/25668-03-corolla-p0420-cant-figure-out-what-is-wrong/?hl=catalytic

He's had all kinds of trouble. Fishexpo had a lot to say about the P0420 code in that thread, and said it's rare that the code actually involves the cat or even the O2 sensors.

My brother-in-law, who does all his own car maintenance and grew up in a family that owned a car repair shop, suggested I use a couple of tanks of either Chevron or Shell premium gas. He said those brands have a cleaning additive that might help clean out excess carbon build-up. He also suggested that when getting on the freeway, to accelerate more aggressively for a few days to help with the cleaning as well.

I'm assuming there's no need to do anything at this point. My next service will be due in a couple of months and I'll have the dealer check to see if the code is still there. My service advisor is very good and I'll see what he has to say.

Is there anything else I should do now?

The CEL might be out, but the code is still stored in the ECM's memory. For most smog stations, a stored code is still grounds for automatic failure.

You are correct that the cat shouldn't just fail just out of the blue - the only thing that you have against you is the mileage on the car. The OEM catalytic will last a surprising amount of time, but won't last forever. You are approaching the mileage where the catalytic converter might actually be slowly dying. But from what I've seen from most of the 9th gen models - more likely a downstream (after cat) O2 sensor is the actual culprit - assuming that you didn't have something more common - like a bad tank of gas, fouled plugs, dirty injectors, or excessive carbon deposits in the combustion chambers.

The way you are addressing this issue is exactly how I would do it. Verify if the code is still stored in the ECM memory, if it is, clear it. Fill up the car with some decent gas and run the nuts off of it on the highway. Fuel system cleaners that have a decent amount of the PEA additives, could also be helpful. See if the P0420 code comes back or not - if it does, then you'll need to do some additional digging.

Getting the actual numbers for HC, CO2, and NOx would be very helpful. As you'll already picked up - the P0420 code is one of those DTCs that could have a number of possible culprits and very rarely does it actually indicate a problem with the catalytic converter. See what the service advisor finds on the ECM - see how many times the P0420 code popped up. If it was just this once, likely it was a tank of bad gas, dirty injector, or excessive carbon deposits.

Thank you, fish. I appreciate your observations and advice. Very helpful.

How old are your sparks plus, both oxygen sensors, and the exhaust manifold flange gasket (donut)?

Also, have you replaced your leaking intake manifold gasket for revised orange silicone gasket # 17171-22060 ?