I have a 98 ford mustang 3.6 and my check engine

How do you know the gas cap did it ? If the check light is on their are DTC'S - diagnostic trouble codes stored in the PCM - engine computer . Your 98 mustang can store hundreds of DTC'S ! You take it to a qualified repair shop and have it diagnosed .That's how you have check engine light turned off .

Depending the style. Go to google. type in 1998 ford mustang fuse box diagram and search for images.
Locate the closest one to your fuse box and look on label for "fan"

I’m happy to help further over the phone at https://www.6ya.com/expert/zelio_00a80ed57645ccbf

3 digits. follow this link for more info. Good Luck.
http://autorepair.about.com/library/ts/obd-i/bl-dtcs-98.htm

okay heres what causes not to kick in,
1 a bad motor
2 bad tempeture sensor
3 bad/defected fuse or relay.
check the fuses and your fan relay first. swap them around to test is every thing well then test the motor dirtectly, if its fine and works, then tempeture sensor on the engine. remove the harness and put a jumper wire on the prongs and what it kick the fan in. have the ign on with engine off for this. if it works replace the sensor.
hope this help you.

send mr your comments thansk

the headlight knob is also a dimmer for the lights on the dash. try turning it all the way one side, then check fuse if still not working.

Are you getting a check engine light?

Try out this link to get a solution. also offers live chat for help
reliable-store.com
all the best!!!

fuel pump, regulator and filter need replacement.

You need to have it scanned to see what code turned on the light. Check your gas cap, make sure it is tight.

I too had this problem. the mechanic at the local dealership said it was no big deal and for me to let it go. I changed my plugs and wires but still a miss. later a had all the catalytic converters cutt off and my problem dissapeared.