I own a Pontiac Grand Prix 2001 with a 3.1 engine. It has 70,000 miles on it. About a month ago the service engine light came on. I took it to an auto store and the code reads “evaporative emission control system leak detected (small leak). They said it might have been an open gas cap so he cleared it. Now it came on again, so is this something that is hard to track down?

I own a Pontiac Grand Prix 2001 with a 3.1 engine. It has 70,000 miles on it. About a month ago the service engine light came on. I took it to an auto store and the code reads “evaporative emission control system leak detected (small leak). They said it might have been an open gas cap so he cleared it. Now it came on again, so is this something that is hard to track down?
Welcome to AF, if not the cap the diagnosis gets tougher ,any small vacuum leak in the system, cracked line, bad vent/purge valve, gas tank sealing can cause this code, a professional smoke machine or step by step diagnostics may be required here.

You can start by replacing the gas cap, not too expensive and easy to do yourself. Also, make sure you always tighten the gas cap to at least 3 clicks, to insure a proper seal, and never leave the car running or have the ignition in the "on" position while re-fueling. Not only is this a fire hazard, it will continually cause this code to trip. If the above doesn't help, I agree with Max that a professional smoke machine is your next step to trace the leak.

So when I buy a gas cap, should I buy a generic one or a factory cap?

Mine has been giving me the exact problem too, keeps making the SES light come on, my mechanic told me it could be a sensor solenoid for the Evap or something? i dont know about that though because every now and then i'll smell gasoline around the car, so thats probably whats setting mine off. i just bought a generic gas cap from advance auto parts, since when i bought my car it diddnt have a gas cap at all. sorry to butt in once again.