I replace o2 sensor upstream should I replace sensor downstream too?

Question

P0421 Mitsubishi - I have a Mitsubishi lancer 2002 and having code po421. I replaced the o2 sensor upstream so far code did not pop up. Should I replace the o2 sensor downstream too?

Answer 1

P0421 Warm up catalyst efficiency below threshold

At one time P0421 was part of a recall , where the PCM was to be reprogrammed which caused the catalytic to melt down.Recall EMR -05-001R2. The catalytic convertor is a mechanical device that needs to go from rich to lean, This is the job of the front 02 sensor. As long as the catalytic is not degraded due to mileage you may not need to replace the rear 02 sensor ,the rear 02 sensor is known as downstream or catalytic monitor sensor. Let it go through its monitor ready  (if you recently cleared the CK ENG LT).