P0421 Mitsubishi - 2.4L intermittant

Question

2004 Dodge Stratus Coupe 2.4L Mitsubishi L4 engine 143K miles P0421 intermittent new catalytic and new upstream Bosch O2. Its either PCM calibration specs, or ???

Answer 1

P0421 Warm up catalyltic efficiency below threshold

Is there any exhaust leak prior to the catalytic convertor? If you have a scan tool that can read the upstream 02 sensor which would be  Bank 1 sensor 1 and compare it to Bank 1 Sensor 2, this would give a good indication of what is going on. Spark plugs condition has to be good, no intake leak that would cause a lean code condition

 Here a good catalytic convertor, being above 0.650 volt would make it a good catalytic convertor

 Here a bad catalytic convertor, which is below threshold 0.650 volts. this is a ineffective catalytic convertor