The van is missing, what could it be? on 2002 Pontiac Montana

We changed the intake gasket, head gasket, the coil packs, and modual. We had the spark plugs changed but did not put the platinum ones in. The heads are not cracked.

Asked by for the 2002 Pontiac Montana
It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. It could be ignition related (plugs/ plug wires breaking down), could be fuel related (lack of fuel delivery, fuel quality or bad injectors), could be a vacuum leak, or an engine mechanical issue so many things it would be wrong to guess.
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If you have a miss you should have a check engine light on. What is the code?