Ok...So I've been putting off fixing my O2 Sensor and decided I might do it this weekend. Though I'm confused with all the Upstream, Downstream, Front Side, Rear Side. Can someone please tell me which one i'm supposed to get. I'm trying to replace it to see if it will fix a P0420 code before I go and replace the cat. Thanks...

 

P0420 is catalytic converter efficiency fault detected by rear O2 sensor. It is not a sensor fault, but is possible to be caused by faulty forward sensor. Code is set when ECU detects the rear O2 sensor oxygen level (voltage output) is swinging hi/lo too much, similar to foward O2 sensor.

It could be the forward O2 sensor is causing this problem, but also can be caused by exhaust system leak ahead of rear o2 sensor. Of course the catalytic converter may be bad also.

Suggest shaking the converter vigorously first. If it rattles badly due to loose damaged matrix inside the converter, the code is probably correct.

Someone w/ a advanced Obd2 scanner (function monitoring available) can tell if forward and rear O2 sensors are working correctly and save replacement. My guess is they are working.

Suggest reading posts on this site for additional background. This is a fairly common problem, and my sense is it is rarely fixed by replacing O2 sensor.

good luck

 

I've been told that by 1999 the ECU is pretty good at "knowing" the difference between a bad O2 sensor vs. a bad converter.

Still, look for exhaust leaks.