Check engine soon light comes on?

JT, what codes are you getting? You don't say year of truck, but I'd guess from your description of the problem you are getting codes for fuel system vacuum leak. May be the gas cap, that's always the cheapest place to start. If the codes are indeed for vacuum loss in fuel system, then the smoke test is the next step. It can take 50 miles or more for the pcm to complete it's emissions checks, a full drive cycle as it's called.

Check the gas cap. If it does not seal tight, you leak gas vapors into the air. Some cars can detect that and use the Check Engine light as the indicator.

If the car has less than 50,000 miles on it, just take it back to the car dealer. They are required by law to keep cars within pollution limits.

Check what kind of gas you are putting in. Some cars can not handle ANY ethanol-gas mixtures.

What's your mileage? I had this happen with my truck and took it in to be checked, and the service department told me that the newer models have this preset by the manufacturer to come on around the time you're due for a major check (50,000, 100,000 miles). Check your owners manual to see when your vehicle has recommended tune-ups. The light may come on a little before you get to that actual mileage so you schedule an appointment before the mileage actually hits the mark. You may not have anything wrong.

A good guess would be to check the 02 sensors. After a few minutes they get hot. If youve recently replaced them and someone touched the tip of the 02 sensor.. it would do that. or they coud just be old. you didnt say how many miles (or KM) were on your car.. but I would look there, first.

take it to the dealer or a chain part/repair shop(Pep Boys etc.). They will hook it up to an engine diagnosis system and tell you what is wrong. Often for free.