shafner30

so I changed my front O2 sensor do I need to change my rear O2 sensor the one after the cat ? would the rear O2 sensor cause the engine not the pass smog in California ? it's a 1990 22re

millball

The rear O2 sensor does not contribute to engine fuel management. Its purpose is to determine the function and efficiency of the cat.

If there are no codes related to the rear sensor, or the rear sensor reporting deficient cat operation, you should be OK.