it might be your throttle
why 2003 ford focus stall when air conditioner is turned on
Guess: Best Guess is defrost or AC are on and the engine is having trouble running the compressor. This is common on these cars when they have another fault with the way they run. Second best guess, The engine has a run problem, and the extra load causes it to be worse.
It is not supposed to be on when engine is running unless there is a problem.
When a check engine light comes on, it means that there are issues with the engine. It gets turned off when the person resolves the problem that caused it to turn on.
Has it always been like this or not? If it is, then it is not really a problem. It is how the stereo was installed. An stereo can be installed to be turned on whether the engine is on or off. If the stereo cannot be turned off at all, then you have a problem with your stereo.
Replace the fuel pump. I had an S-10 that acted similarly - turned out it needed new spark plugs and wires.
leave the ignition turned on for 45mins or until the red security light on the dash has stoped flashing then start beware if the engine stalls or you turn the engine off you have to wait anutha 45mins
On most vehicles, after the problem is taken care of, the check engine light resets automatically after the ignition is turned off and then the engine is started again. If you have fixed one problem, a second problem probably exists.
Scan tool, will give you a trouble code as to why the computer turned the check engine light on.
check the battery and connections/cables
take negative off battery for 30 minutes and clean the Throttle Body
I had the same problem on my 1999 Windstar. It turned out that the light bulb for this indicator was burned out. I simply replaced it and the problem was solved!!