'03 Chevy Impala, Check Engine Light, OBD II Code 420???

[redddogg60]

OBD II , Code 420 (Catylist efficiency below threshold, bank one). Is this a catylitic converter problem, or a Oxygen Sensor problem, or something else??? Has anyone experienced this before??? Thanks in advance for any help you may offer.

[Iflylow]

That rear O2 sensor is the one whose signal is used to determine cat function, so that might be a place to start. Usually a plugged cat will show up as poor acceleration due to the high back pressure. You can do a back pressure test to see.

[BNaylor]

Originally Posted by Iflylow

Usually a plugged cat will show up as poor acceleration due to the high back pressure.

Possible but not necessarily. The CAT convertor can still have less than desirable oxygen storage capability, not be clogged/plugged and still trigger the P0420 DTC . The rear 02 sensor is most likely working properly since it is able to detect the CAT convertor low efficiency problem. In most cases P0420 indicates a CAT convertor issue so it may require replacement which may be covered under GM's emissions/convertor warranty depending on the mileage and age of the car.

Also, it would help to post which engine and mileage.

[redddogg60]

This vehicle has 66k miles on it.

[BNaylor]

GM's CAT convertor warranty is 8 years or 80K miles, whichever comes first so get it into the dealer for warranty work.

[De-railer]

Yep have a dealer check it since it's still under warranty...they can do a back pressure test to see if it's clogged. It's sort of a common issue on these cars.

[redddogg60]

Thank You For all the info.. Will make an appointment at the dealer. I'll update everyone with progress or lack of. THANKS AGAIN!!!!

[redddogg60]

Converter was the problem. Covered under warranty 'til 80,000 (certain models are covered to 10 years/120K, per dealer). Thanks Again!!!!