I have a 1998 truck that decided that it didn't want to run today. The last OBD code from it was P0422 "Main Catalyst Efficiency Below Threshold (Bank 1)" To me this sounds like a catlaytic converter issue which should be covered under warranty. Can any of you share experiences/expertise to help me not get the wool pulled over my eyes at the dealer? The truck starts but idles intermittently, as if the motor is being turned on and off. When I depress the gas pedal, I get more RPM's, but still an intermittent on-off kind of thing. Any help is really, really appreciated.

 

I would be surprised that a faulty downstream O2 would cause it not to run, have you checked over all the other basic things. The downstream O2 switches at a slower rate than the upstream, as the efficiency of the cat declines the switching rate of the downstream gets closer to the upstream, once it reaches the limit a code is set.