Reynaldo P0300 Random misifre P0446 2003 GMC Yukon

Question

I was going to loosen exhaust flange per your suggestion to determine back pressure and found that the catalytic convertor had been replaced with a piece of exhaust pipe. Both O2 sensors are still there and hooked up. Thought this info may be of interest as to help with the diagnosis.

Answer 1

P0300 Random misfire

P0446 Evaporative emission control vent control circuit malfunction

There a reason for the exhaust pipe to have a straight pipe, and all the more to further the investigation for exhaust back pressure. When a catalytic convertor has to endure a prolong misfire condition it can shatter apart and pieces can go into the exhaust system and somehow get stuck in there,

 Here a catalytic convertor that endure a misfire the result was a few pieces still in the catalytic convertor. The rest was stuck in the muffler, The vehicle came in with a P0101 and a P0420 ,P0430 with a rough idle. This is all that was found ifor the catalytic convertor. This is a Chevy vehicle

 Here the catalytic convertor. The rest of the material of the catalytic was stuck in the muffler, which resulted in excessive back pressure.  This is a Chevy vehicle

 Here a Lexus where the catalytic convertor was removed and pipe in place for both sides. Same thing , The vehicle ran great at idle, but at highway speed , and coming into a stop. it had low idle with a misfire condition. Catalytic material was stuck in the exhaust pipe. as a fact standing behind this muffler you can feel pieces shoot out.