Celica Forums General Discussion Engine and Performance Check Engine Light lingering problem

hey so i have a 2002 toyota celica GT. Every year when it gets cold, my CEL appears with a Po420 code. and once it gets warm outside is disappears. well i need to get my car inspected and i cant do so with this cel. did some research and saw that i could be an old catalytic converter or a faulty O2 sensor. Before i take it to get diagnosed at an auto shop, i wanted others opinion. since it is temperature related, wouldnt it be just a sensor? and if it is an O2 sensor how easy would it be to replace. Thanks

What state are you in?

The P0420 is Cat converter inefficient. It does not clean the exhaust enough. The aft O2 sensor behind the Cat reads the Oxygen content and compares with the fore O2 sensor before the cat, if it sees relative high Oxygen content, it will flag the Cat as not working, even in borderline cases.

if you dont want to replace the expensive Cat, you can add an extension (14 mm tube) to the aft O2 sensor. It will read less O2 content and the ECU will think the cat is working normally and turns off the P0420.

Check with your state emission regulation to see if it's legal to modify the exhaust to pass smog. Smog shops are supposed to check for any modifications to the exhaust, but they rarely do and likely wont know if they even see it.

thanks man. just ran some tests and all of my O2 sensors were normal and “passed”. would the de fowler work?

Would get rid of the P0420.

To add or replace the O2 sensor, jack the side of the car on passenger side up about 12 " and secure it on car stand. That would give you enough room to get under the car and access the O2 sensor on the pipe.

The pipe is extremely hot and sensor may be rusted or stuck. You may have to spray some liquid wrench on the threads and wait to loosen it. When putting it back, apply some antiseize high temp grease on the thread so it will be easy to tak eout next time.