replace 02 and cat on 09 saturn outlook still have ck eng lite

Question

check engine light still comes back on

Answer 1

The 02 sensor that reports a failed catalytic is the downstream 02 sensor, The front or the upstream is the engine management sensor, If the front is functional the engine is in fuel control. If the rear is non functional it will set a P0420.

​ A typical failed rear 02 sensor, the ECM reported a P0420, viewing the rear 02 sensor with a scan tool that give waveform, this is a failed downstream 02 sensor , which set a false P0420.