Code P0420

[acton mike]

a bit of a storey

my son's 1998 corolla is due for its every-two-yeare emission test

I get him to bring it me for plugs, air filter, oil change - all that good stuff

I see the check engine light is on

"How long has that been on?" I ask him
"I don't know" was the answer - i suspect he does not care either

anyway on goes the OBD2 tester
code P0135 O2 heater circuit bank 1 sensor 1

sure enough a broken wire on the harned - the engine side of the plug

broken a fractin of an inch away from the plug making the repair a proper bugger

fix that

clear the code

take if for a drive

check engine light comes on again

P0420 - according to the tester screen - this means Catalytic system efficiency below threshold bank 1

I suspect that the cat is probably cooked from the car running rich as a result of the front O2 sensor problem

Would i be right in that assumption or should I look for something else that will set the P0420 code

It is the only code stored

[fishexpo101]

That is probably a fair assumption. Only way to find out for sure is to backprobe the upstream and downstream O2 sensors and see if the signal waveforms look too much alike.

The P0420 code is set if the ECM measures waveforms that look nearly identical from the upstream (pre-cat) and downstream (post-cat) sensors. Assuming the sensors are working perfectly, the only possible culprit is a faulty catalytic converter. Possible culprits for setting the P0420 code, are bad O2 sensors, bad catalytic converter, exhaust leak, or faulty ECM.

Try resetting the code and drive normally. If the P0420 comes right back - investigate the problem further. Sometimes it isn't even related to the exhaust/emission components - as I've seen other people with a P0420 code, replace both O2 sensor, catalytic converter, and replace the entire exhaust still end up with a P0420 code. Turns out it was a engine coolant temperature switch that was bad. Each case will be a little different.