Yesterday, I took a trip to the mountains, about 7500 ft. elevation. This morning, I start the truck and the check engine light is on, never happened before. Truck has 12k on it. Re cyled it a few times, light still on. Drive about 1 hour and the light remained on. I had some work done on the truck today, shocks, air bags, and it was off for about 4 hours. When I started i,t the check engine light is now off.

Did the drive up to high altitude have some affect or is this just some little gremlin?

 

I'm not sure what you mean by "Re cyled it". Did you pull the codes? Without starting the engine, turn the key to the On position, then off, then on, off, on. Do this within a couple of seconds. The engine codes will show in the odometer as Pnnnn where nnnn would be a 4 digit number. When it is finished, it will show Pdone. Post the codes here. If there are no codes (only Pdone) then it cleared. Wait till it shows up again and now since you know how to pull the codes, pull them and write them down.

Might be a gremlin, might be something minor that reset itself after you restarted the engine a certain number of times.