How much does your service engine light being on bring your gas mileage down? Doesnt it put your ECU into safe mode or some thing? Causeing the car to run rich?

 

Depends why the light is on.

The light in and of itself doesn't define how the PCM reacts. e.g. - if you have a problem with the EEC purge solenoid, it affects nothing, but turns the light on, because its an emissions problem.... not "safe" mode, not "limp home" mode, because it isn't required. But another example, if your knock sensor goes bad, the PCM defaults to a calculated timing retard program, and you lose performace and a bit of fuel mileage. Or, if your SES light is on for a "bank 1 lean" and the cause is a false lean due to exhaust leaks or misfires, the car is going to run real rich and the gas mileage is going to go in the crapper.

If the light is on, it pays to find out why.

 

You just gotta love the intelligent responses you get here! The light is on for an EVAP problem. I already replaced the gas cap and reset the computer and it came back on. What else can the EVAp code be on for?