2001 Oldsmobile Bravada Check Engine Light and Fan

I just purchased this vehicle about 6 weeks ago. 2 weeks after I bought it the check engine light came on. I took it to my mechanic who tested the computer and said that it came on due to me not closing the cap tight enough. I also had them check the heater as it was only blowing luke warm air and never gets hot, also the air will only come through the floor vents and defroster warm, if I turn it to hand/face vents it feels like it is coming from the a/c. They reset the computer and put a new thermostat in the vehicle and said they unplugged one of the vents. The check engine light came on again yesterday and has not/will not go off and the new thermostat didn't change a thing. I know it isn't a problem with the cap being tight enough because I turn it 3-4 times around and my tank was over half empty this time when it came on. I would like to have these problems fixed but I hesitate to take it back to my normal mechanic since it doesn't seem to have done any good the first time. Any suggestions on the problems I might be having?