Originally Posted by TerminatorX5

no, a slight misfire, or retard of the spark will not set off check engine light... severe misfire, misfire under heavy load, several cylinder misfire will set off the light... sometimes it is not a misfire per se, it is just engine roughness, as monitored on the live feed from the engine that can be first sign of impending trouble... the things about the cats, since they are downstream from the main events, they can be a casualty of the problem upstream... of course, there is a loop back, where the info from the O2 sensors and the cats is fed back to the front, thus creating a vicious circle... as long as you have rectified the issued upstream and downstream, you should be good...


listen to your engine while it idles, without any loads, no AC, no seat heaters, no radio - just plain engine and only normal systems...

as you listen to the engine, the engine noise should be even, without any extra over- or undertones that fade in and out... if you hear them, that means that the software is making adjustments in spark, timing of the valves, blah, blah blah... some very light varioations are natural, due to varying fuel pressure (still, pressure should be constant), and the fuel quality - but if you use a 1st tier fuel supplier (do a search on this forum for 1st tier fuels), after couple of full tanks of gasoline, your engine should just purr... if not, then hook up live monitoring software and see what is going on there...