I agree with Dave and sean,

The ECU uses Crankshaft acceleration to determine if a cylinder or cylinders are down on power. This does not mean an electrical misfire but could be anything.

Some GM systems use spark ionisation to determine "cam position" (cyl ID) which I think is also capable of detecting misfires.

Many garages assume coil faults when P030x codes are flagged but don't even test or confirm if they are at fault. I had another one last week where a garage had replaced a coil due to a misfire code and car was still the same so they gave up and sent it to me. I tested it and found the new coil they had fitted was faulty.

Which begs the question, if they can't test a faulty componet, how can they confirm if that is actually the fault or not when duff parts are fitted? The worst ones will carry on fitting duff components elsewhere making my job harder as I have to fix what they have broken before I can get to the original fault.

Or the other scenario is they are convinced the new parts are faulty and send them back to supplier demanding a new set when the fault actually lies elsewhere.

As stated earlier in this thread the poor quality of parts out there now being supplied by the likes of ECP and now Andrew pages trying to compete are selling cheap garbage does not help matters.

Mike