My 2008 Lancer ES keeps throwing the P0421. I know this refers to either a bad cat, or bad sensors mostly...but, the CEL throws for maybe...a few hours (or days) and then it goes away on its own (While the car's still running, so not just after turning it on)

I guess my question is.....is that more of a clear indication of an o2 sensor? Since it seems to keep turning on and off randomly?

 

Is the car stock?

If so - yes, this would indicate a failing front O2 sensor to me.

 

Yes it is stock....another thing that just popped up....i'm getting the P0420 error now, and not the P0421 (Which I was getting)....but again, these codes pop up for a little bit then disappear on their own randomly. Confusing =/

 

The p421 seems to be a Pzev only related code. Do you have a Pzev car?

I would suspect the sensor. The reason it comes and goes is that the OBD monitor continuously runs and when it meets the requirements long enough, it will clear the code.

So, inspect the wiring and make sure it is good, no rub thrus or corrosion on pins. Then replace the cat outlet sensor.