smog and check engine light

[ca_mustang]

just failed smog! (in california)

check engine light doesn't stay on when the key is in the 'on' position but the engine isn't running. (1987 LX Auto 5.0)

could it be the ignition switch?

ignition module?

fuel gauge also always says full, is this related.

HELP!

[bigred90gt]

You should try to pull your codes. I copied this off of one of my other post because I'm too lazy to re-type it. It's the cheapest way to pull your codes.
As far as pulling the codes, there is a self test module inside the engine bay, all you need is a jumper wire about 5" long with 1/4" blade plugs on each end, and something to write with, if you have a check engine light. If so, start the engine and let it run until at normal operating temp. kill it and wait 10 seconds. In the engine bay, right next to the windshield wiper motor, there should be a harness with either 5 or 6 slots. 2 on top and 3 or 4 on the bottom. There should be a little jumper wire with a single slot in it just to the side of this. Plug the jumper you made into the top right slot on the harness and plug the other end into the jumper with one slot. Turn the key to the on position but do not start it. The check engine light sould look like it is shuttering rapidly, these are codes used by the dealer. After a brief pause the light will blink, pause for 1 or 2 sec and blink again. These are your KOEO, key on engine off, codes. For instance, if the light blinks 3 times and pauses then blinks 4 more times, this is the code 34. It should repeat any codes and all codes. If you receive a code 11, this means that no problems were detected. There should then be a 6-9 sec pause, a single flash, then a pause of another 6-9 sec and then another set of codes. These are your Continuous memory codes. Record all of your codes and then post them. Someone should be able to help you with them. There is also a KOER, key on engine running, test. You should correct any problems from the KOEO test prior to running the KOER test. corral.net has an indepth explaination and definitions for the codes. I think that depending on the year of your car, you may get 2 or 3 digit codes. I have a 90, and I received 2 digit codes. I hope this helps.
I also have attached a picture that shows the harness you need to jump.

[PKRWUD]

When you turn the key on, but don't start the engine, the MIL will come on for about 5 seconds, and then go off. This is normal.