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1999 Malibu Code P0420

01-08-2013, 10:17 PM
Does any body know why this OBD code keeps poping up on my Malibu
IT keeps telling me bank 1 i changed the bank one O2 sensor run the car 34 miles same code pops up agian. I was told to changed the down stream sensor so i did drove the veh i got 65 miles then check engine light came back on giving me the same code P0420. Now im being told its the cat,,, . Any ideas out there im all ears lol . I need to smog my car. but first I need to get rid of that check engine light. Thanks

01-09-2013, 10:29 AM
95% of the time, it's a bad/blown catalytic converter, see

Tech II
01-09-2013, 02:02 PM
Whoever told you it was the front O2 sensor, doesn't know what he is talking about....a bad front O2 sensor would set it's own code....

P0420, is a catalytic efficiency code.....

The PCM, or computer monitors the front O2 sensor, and the rear O2 sensor....when the data for BOTH O2 sensors "mirror" each other, that means the cat is no longer working.....when the cat is working, the front sensor is rapidly changing high and low...the rear O2 sensor should will be high for awhile and then be low for awhile, depending on whether the cat is storing or using oxygen....

This can be seen with a good scan tool, by plotting both O2 sensors.....if the rear one looks the same as the front one, this is the term "mirroring", then the cat is no good....

01-09-2013, 05:09 PM
Tech II is dead on,textbook too. The rear sensor also makes changes slower than the front sensor.

01-09-2013, 06:45 PM
Another way to test the cat is to measure the inlet vs outlet temps at the cat, after a drive cylcle to fully heat the cat, the outlet temp should be at least 100 degrees higher than the inlet, use a infrared thermo. If not it is bad and will set a code.

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