I have a 2000 z28 it is throwing 2 codes , p0420 catalyst efficiency below threshold (bank 1). I replaced the 2 oyxgen senors before the catalyst. It seem to make it run better but i dont know wat else it could be???

 

P0420 indicates the the AFTER-cat O2 sensor is not seeing the change in O2 content that the catalytic convertor should be making. Indicates either the AFTER-cat sensor on Bank 1 is faulty, or the catalytic convertor is deceased. Swap the AFTER-cat sensors side-to-side. If the code stays on Bank 1, its a cat problem (or possibly a sensor wiring problem), if the code follows the sensor, its a faulty O2 sensor.

Why would you replace both BEFORE-cat sensors to try and solve this problem? Its related to the AFTER-cat sensors, and you only have the problem on Bank 1.

 

it was throwing a p03 code before, when i had the code cleared it stayed off for a while, then it started throwing this one. Thanks for the info

 

If the p03 you're refering to is P0300 random misfire then you should repair that before trying to fix the P0420. An engine in need of a tune up affects cat efficiency. I always make sure the engine is running up to par before chasing cat efficiency codes. Another thing is if theres any exhaust leaks it affects the o2 signals.

 

I got it fixed it was the 02 senor behind the cat on the driver side. Thanks alot guys