Engine light and shuttering/stal at stop.

My engine light came on in the Focus and everything was running fine we just had a tune up and they changed the fuel filter. Once the engine light came on every once in a while the car would start sputtering I guess you could call it and then rev back up but it starting to stall every so often so we took the beast in and had them hook it up to the diag machine and they said it was the gas cap was not tight so we paid $39.00 for them to check the car and tighten the gas cap for 1 day the light went off now its back on.

They also said I needed a tune up just got one a month ago and I need to replace or clean the fuel filter can't remember which one he said.

I just want to know if anyone has an idea of what might be wrong does the diag machine tell them it's the gas cap or just air in thr fuel line? If it's air in the line could it be the fuel filter we had changed or the fuel injectors like they said needs to be cleaned or replaced. Or do I just need a new gas cap?

Any help would be great.

I had the same problem with my 02 SVT focus. I took it in to Ford and they reprogramed my ecu and poof no more problem. Well almost. After getting them to re-program my computer I installed a cold air intake. Every once in a while I get the ocational sputter. So if your going to any engine mods do them before you reprogram.