Yesterday out of no where, the Check Engine light came on and is still on. I also saw the tack go to zero while at highway speed for a few seconds, then returned to normal. Everything continues to run well and all indications remain normal. I have not made any additional performance enhancements or done any maintenance for some time. Do only emissions related items trip the MIL or are there other things also? What is "limp-in" and items associated with such failures? Any ideas of what to look for? See signature for truck information. 105,000+ miles.