Check Engine Light on!!!


New Member
Sep 23, 2004
On my 1990 LX 5.0 the check engine is on. I disconnected the battery to clear codes and as soon as I reconnect and turn the key the check engine light comes back on and stays on. Any thoughts on what it might be??
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Without any description of any symptoms, it's going to be kinda hard to diagnose. Assuming that the car isn't behaving like anything is wrong (since you would have mentioned it if it was, right?)
A) For that era stang, there may be nothing wrong with it- that's why they're called "idiot lights".
B) Something is wrong and it is popping a constant code, take it to a good technician and have them run the codes and tell you what's pooped.
C) Something is wrong and it is popping a constant code and you don't want to pay a professional to tell you nothing is wrong, so refer to jrichker's plan for running codes yourself using a paper clip. Sorry, don't have a link, but if you pm him, he'll give you the rundown- or somebody else may pop up with it. It should be a sticky by now, IMO...
Well when I bought the car last October the check engine light was on. This winter I had the battery out and fixing it up and today I put the battery in and of course the car won;t start but the check engine light is on. In january the car started fine. When I switched the carpet I noticed the wire on fuel pump relay got ripped out so I'm thinking that's why the car won't start. I don't even hear the pump running.
No the check engine light was on from the moment I bought it. I drove it home and drove it around the block for a couple of months and the check engine light has always been on. How do I do a self test to see what codes come up?
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.

See for more descriptive help

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 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.