My mom's 2003 Alero has had a bad catalytic converter for the last few years.. at least that's what she was told when she took it to the shop. The engine light (P0420 threshold below efficiency) has been on over the last few years and I occasionally check it with a code reader to make sure no other codes have been added. Up until today - as far as I know - none have been. Now today when she was driving it the engine light started flashing and running rough and making a knocking sound.
She brought it straight home and I hooked my scan tool up and it gave the same cat code as I just mentioned, plus a P0306 misfire code. So I pulled the plug wire on number six cylinder and hooked up a spark tester to it - and it showed a decent spark. And to verify the spark being normal, I tested at another plug wire and the spark looked about the same..so I don't think anything is wrong with number 6 plug wire or coil or anything like that but am not sure. Also, the spark plugs were replaced a couple of weeks ago because the ones in the car were worn down about .010''. I can't imagine this having anything to do with the misfire because I did them myself and made sure they were gaped properly and that all the wires were put back on the correct plug/cylinder and plus it ran fine after the plugs were installed - but thought I would mention it anyways. And I was very careful not to damage the platinum on the plug when gaping them.
Did a little bit of research and found this: A code P0306 may mean that one or more of the following has happened:
Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
So where should I start? Any ideas?