There isn't enough information about your car to give the best answer. The wiring and computers changed several times between 86 & 93.
That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.
Dump the codes and see what the computer says is wrong…
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.
See the sticky "Help me create the Surging Idle Checklist" at the top
of the forum for help with your idle problems. The first post has all
the updated answers.
Note that all resistance tests must be done with power off. Measuring resistance with a circuit
powered on will give false readings and possibly damage the meter.
Most no code problems are due to wiring problems on the computer pin 46 signal ground wiring.
Pin 46 on the computer is a black/white wire that connects to power ground inside the computer.
Use an Ohmmeter and measure the resistance between the black/white wire on the computer
self test connector and the negative post on the battery. You should see less than 1.5 ohms.
More that that means wiring problems in the form of broken wiring or bad connections.
The other wire to check is the white/red wire (marked STI in the diagram). It is a single wire
with a gray plastic cover over the connector. Measure the resistance between it and pin 48
on the computer. You should see less than 1.5 ohms.
Using the above diagram, you will find pin 30. Normally problems with pin 30 cause a code 67
and you can't get the computer to go into test mode with the engine running. Grounding pin 30
to pin 46 can be used to trick the computer to go from Key On, Engine Off test mode to
Key On, Engine Running test mode. But first you need to jumper the self test connector
properly to go into test mode.