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Which cylinders are misfiring?
The problem with these is that sets even if one is misfiring and you need a scan tool to determine which one. Possible issues are intake gaskets leaking, fuel system issues, just to name a few.
An intake gasket leak would not be something you see, but something that you can tell by spraying carb cleaner or ether around it.
The gas Cap would not cause a misfire.
It may be missing at a time when you cannot feel it. ALmost every motor out there has an individual misfire every few seconds, but one only and you will not feel it.
You changed the coils, but there could be a cylinder head issue, injector, wiring, fuel supply, exhaust restrictions, mass air flow sensor, and the list goes on. The light is on because there is a misfire, and that can be caused by anything that prevents the engine from firing properly on the cylinder or cylinders in question.
THe code reader? Because it is only that, a code reader. YOu need a scan tool to determine which cylinder is the issue.