We would need a little more info, to be able to try to help you.
What year truck?
Have you had the codes pulled?
42,000 is very low mileage for C or K truck. What model truck is this?
If you go to a local Autozone, PartsAmerica/Advance, VIP Auto... they will be able to help you pull the codes.
Could be something as simple as an O2 sensor. Any issues with the engine, transmission lately?