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Q: Misfire

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so i have been having issues with my car, first the battery died, easy fix. then the valve cover gasket and all the plugs where changed. then i had the exhaust completely redone. now it is intermittently misfiring on all cylinders. some days it'll drive fine, some days it will misfire on all cylinders. some days it'll be driving then start to randomly misfire. I've checked the plugs, I've changed the fuel pump and the filter. all the spark plugs are new. I'm wondering if it is my ear valve? I've also sometimes get a catalyst efficiency low code. so maybe the ear isn't giving enough back pressure to my intake?

My car has 70110 miles.

Hi There, The catalyst efficiency low code (usually code P0420) relates to the catalytic converter typically failing itself. There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other possibility may be the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors or a plugged catalytic converter.

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