I have a 1999 Protege ES 1.8L engine with 104,000 miles on it. Thecheck engine light came on and showed code P0420. What is the most common cause of this.I don't know how to check the downstream sensor, only that is must be that or the Cat. Engine runs good andseems to get the same gas mileage as normal. Only problem I've had with the car other than normal wear items.

 

the most common cause is your front CAT. you can test it by monitoring your front and rear O2 sensors. your front sensor should oscillate from lean to rich, with your rear sensor staying constant. most times your converter will be working, just not fast enough to satisfy your pcm's parameters. most likely you'll have to replace the CAT to get rid of the light