P0420 Tripped Only When Coolant Temp Getsto 175F

All that oil burning as engine ages, combined with exceeding oil capacity during oil changes when car was young, is what fouls the cat and prevents it from working properly. But the good news is an aftermarket replacement cat only costs $130 online and your old clogged cat can be sold for $1500 because of the precious metal inside of it. So truth is you can actually make money by fixing this problem.

 

You can replace spark plugs, clean throttle body and MAF and do an engine flush treatment before an oil change and then reset 12v to get the engine burning a little cleaner and that could get the problem solved.

And engine temperature is not the same as Cat temp, which heats up sooner and faster. Once engine hit 180' and thermostat opens engine does burn cleaner than at cooler temps. But the cat on the other hand doesn't throw an error code until cat reaches optimal high temp and in your case you have a bad cat soon as it checks, which is slightly before thermostat opens, but not really that related to it.

 

Thanks for the clarification... Could be the first sign of things going bad for the cat as that's when the engine is still a bit cool and not burning as clean. Could also be you bought some bad gas. Maybe add some fuel treatment and run a couple tanks of gas from a different gas station and see if it goes away?

 

The P0420 is what is known as a "2 trip code". The engine computer has to see consistent failures occur during 2 seperate drive cycles. The first time it fails it sets a "pending code", which is stored but doesn't turn on the MIL / CEL. After a cold soak and warmup the ecu will run the catalyst monitor test again and if it fails then it will turn on the light.

Each of the monitor tests has what are known as enabling conditions- things that have to occur before the test will run. One such condition for cat monitor is that the engine reaches a minimum operating temperature.

As far as fixing the problem. First I make sure there are NO exhaust leaks from the cylinder head to 18" past the downstream O2 sensor. None. Not even a pinhole (look and listen closely at the gasket at the spring-bolt flex connection).

Second is to check how the engine runs. No misfires. I watch fuel trim numbers- ideally short and long term should be near 0 under idle and cruise. Things like dirty MAF, throttle body, or injectors can skew the fuel mixture enough to prevent the cat from working properly. Cooling thermostat works correctly. If the car has a fair amount of miles I might just replace the the upstream air fuel sensor and see what it does. Old sensor can be slow or biased which also messes with the exhaust gases "feeding" the cat.

I also check to see if there are any relevant TSBs for this problem.

If it still throws the code then it gets a cat. Just be aware that aftermarket cats are much less $$$ than OE but might not work as well or last as long- or it could be just fine enough.
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I would get a "genuine" aftermarket Denso from a reputable supplier or the OE Denso from Toyota.