I have a 2002 Protege 5 with 105,000 miles. In May I replace both the upper and lower Cat, in July I replaced the after Cat o2 sensor. Each time the check engine light went off. Yesterday check engine light came back on, got PO421 code. From what I understand that is the after cat o2 sensor. Could my new sensor go bad ? How do I check the sensor ? Any ideas would be great, this check engine light has been an on going problem.

Thanks, Chris

 

can't you olm the sensor? ill get back to you on how to do it but im sure you can. and i know how you feel about that light mine turned on so much we took the light out because since i took my cat out it keeps tripping the sensor.

 

k to olm out your sensor you need to bring it to operating tempature them unplug the sensor from its harness and olm the sensor. rev your car up to 3000 rpm and your sensor should read 0.5 olms then floor the gas pedal and it should read between 0.5 - 1 olm then let it go to idel and it should read 0.4 - 0 and if it doesn't your sensor is the problem.