I have a 2000 gmc jimmy. i bought it 3 months ago and ive had no problems with it(other than bad gas mileage). me and my dad were at walmart when we were fixing to leave i started my jimmy it died instantly i turned it over again it started just fine and my check engine light was on. before i left work i put 20 bucks 6.668 gallons in my truck and was in a hurry didn't notice if my check engine light was on or not. i know what the error is i'm just curious if it could be the cause of my bad gas mileage because since i got the truck the only problem i've had is that it gets bad gas mileage ( a little over 13) i wasn't sure if this could have been my problem the whole time with my gas mileage and the light just never came on untill it fully went out

 

I would start off with a fuel pressure and leakdown test as well as cleaning the MAF sensor. When was the last time it had a tune up?

 

the dealership i got it from said they did a tune up when they got it. but here is something new i just noticed which may help narrow it down when i took my gas cap off a little while ago i had some air spew out.