P0420 What else to do

[Rob1965]

Hello all

I have an 2/2001 Corolla with a P0420 code. 120,000+ miles. I have changed the O2 sensors with Toyota ones, All exhaust gaskets and the converter (aftermarket). With a scanner connected___ I see the front sensor switch ing normaly and the rear one mirrors it when cold. Once hot the rear O2 starts to flatline as it should, then the readings start to bounce all around. This sets the code. No other codes present. I have cleaned the Mass air sensor too... anyon have any ideas, Im at a loss now. Thanks Rob

[Bdub215]

Originally Posted by Rob1965

Hello all

I have an 2/2001 Corolla with a P0420 code. 120,000+ miles. I have changed the O2 sensors with Toyota ones, All exhaust gaskets and the converter (aftermarket). With a scanner connected___ I see the front sensor switch ing normaly and the rear one mirrors it when cold. Once hot the rear O2 starts to flatline as it should, then the readings start to bounce all around. This sets the code. No other codes present. I have cleaned the Mass air sensor too... anyon have any ideas, Im at a loss now. Thanks Rob

Try a Factory Cat if you can find one. Some OBD-II cars (actually most i've seen) arent fond of aftermarket Cats.

[sam333]

Its a common problem with alot of cars, even if your cat and sensors are working right.

If you are reading your scanner right( your rear o2 readings are not right ) then its not just a touchy ECM. I would look for air leaks in your exhaust between the exhaust manifold and the rear o2 sensor.

Pressurize your exhaust with a shop vac on blow( tape it to your tailpipe) and then use a spray bottle with soapy water to look for leaks( look for bubbles).

After market cats are not as good as Toyota's but they are much cheaper and work well enough to get past smog, but they dont seem to last as long.

If you have to pass smog and cant have the code showing you can go this route:

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