2000 Chevy Cavalier O2 sensor code

MULLIGANSTEIN

MEMBER
2000 CHEVROLET CAVALIER
4 CYL
FWD
AUTOMATIC
138,000 MILES

OBDII code was: PO420 MOD$10
catalyst system efficiency below threshold
Bank 1
I replaced the downstream sensor but the code came back. Can you give me a lesson on O2 sensors and the Cat so I can try to pinpoint the problem without throwing too many parts at it.
My next step would be to replace the upstream sensor, but it is reading a steady 900 mv.
Also, what is Bank 1 and what does that mean?
What is Mod$10?
Perhaps the first 3 or 4 places to look with regards to this model number, engine size, age, mileage etc.
Thanks very much for your help.

1 Reply

BLACKOP555

MECHANIC
10,371 POSTS

Bank one sensor one is referred to as the sensor before the catalytic converter on the side of the engine that the #1 cylinder is on. It is used for fuel corrections

it should be CONSTANTLY switching between.1v and.9v. If the o2 sensor is getting bad it will stick at a random voltage and barely switch. Sounds like that is whats happening with your vehicle. If the sensor reads improper the bank 1 sensor 2 will read faulty as it compares its readings with #1. If the #1 isnt switching and the #2 is even slightly switching the computer interprets this as a failing catalytic convertor