:hmm:I just swapped out my w4m PCM for my stock version today so I could send it to Jesse to be retuned. Since I got the PCM I've added B&B headers and catback exhaust and also a 160 degree thermostat today while changing the PCM. The exhaust changes never caused the check engine light to come on (with the w4m tune) but with the stock PCM back in place today the check engine light is on and I am wondering if this is because of the exhaust, the thermostat, or all the above. Any thoughts? I'm more curious rather than concerned.