x2, I bought a code reader for my sons car just to have it in there and it was $69 at local parts store. Might be worth the investment, since a shop will charge double that and it could be something really simple if the truck is running fine. If you purchase a code reader, get one that displays the code and clears it, I all ways suggest the middle of the road one, not the cheapest but not the most expensive one either.There are a 1,000 different codes-- Without the code it is only a guess.
Many buy their own code reader- less than $100 at many part stores & Wal-Mart. That way they can also clear codes.