I am hoping for some advice I am working on my Wife's Uncles car, the things I worked on they are fixed. But he also has a check engine light that comes on after you clear the computer it comes back on as the engine warms up.

The code is p0420 I know this means: Catalyst System Efficiency Below Threshold Bank 1

There is two cats on this car, one in the front, and the other about middle ways on car. I believe the problem is the front manifold cat that has four o2 sensors.

What I would like to know for sure is this a code for only a cat issue and not the O2 sensors?

cause wouldn't the code be something like sensor 1 bank 1 if it was a sensor issue?

I also believe if it is a cat problem it probable would be the cat where the o2 sensors screw into it, the one in front of the engine fan and not the one middle ways down the car.


Would appreciate some advise on this problem, cause I am hoping not to have him spend money on the wrong thing and it not being the issue as to why he keeps getting this code....

Thank you in advance...