ok, i have a 2001 monte carlo ls. my engine light keeps comin on and when i read it with the obd2 reader it gives me a code of 0420. which is something to do with my catalytic converters or my oxygen sensors. i had it checked out before and was told that it was the aftermarket catalytic converter placed on the car a little back and it would always mess with the oxygen readings on the O2 sensors. i don't know if this is true or not, but it's costing me in gas and i need to find a cheap fix for this... any ideas?

 

That's a cat code, O2 sensor should throw an O2 sensor code. I've heard that the aftermarkets cause this problem A LOT.

 

X2

The aftermarket cats will throw the P420 code roughly 500 - 1500 miles.

Just get a hand held scanner that can delete it and throw it in the center console to clear it.

 

hey when you figure this out let me know. my 96 is doing the same thing.

 

Or check your EGR Valve, I believe the EGR Valve also throws a 0420 code. Take it off and see if it is carboned up. Clean it if it is. I'm not refuting the cat at all, but the EGR is something to check too. )