SES light help

Hello all, I'm having a problem with my girlfriend's car (2000 Nissan Sentra XE). The "Service Engine Soon" light is coming on and remains so, except when she sometimes turns on the A/C it will go off and then come back on when she restarts the car. At first I thought it could just be a dirty fuel tank so I added a fuel system cleaner but that didn't work.

Has anyone had some kinda problem like this? Also where could I find the computer codes from the ignition key?

Thanks
CamaroZBoy

any chance she likes to leave the gas cap off after filling up?

you can get the code checked at Autozone, I believe they do it for free, or you could take it to the dealer for however ungodly amount they charge.

she may have done it once (it's possible) but not repeatedly.