Its the first GM car Ive been unable to retrieve codes, I turn the key to the "on" position (not start) and immediately the primary (I believe its the primary, its the drivers side fan) fan runs, after I start the car the SES light wont go out (stays lit from the "on" position through starting the car, and while its running. I assume the car has codes to give me, but after I use my actron tool to do the code retrieval procedure nothing happens the light just stays lit, I even tried using a hairpin connecting the 5 and 6 slots (or A and B). It runs erractically (lopes at idle) and its runnin rich. Why wont it flash?

It is normal for the primary fan to come on with the ALDL connector jumped or a scan tool plugged in. Do NOT start the car with the connector jumped. To retrieve any stored codes jump the pins or use the tool and turn key to ON, but don't start it. It will then flash a code 12 several times, then proceed with any other stored codes. You can jump the ALDL connector after it is running, that is called the field service mode. It will then by flashing at different rates tell you if you are in open or closed loop and how your mixture is. Code 12 will flash 3 times, then stored ones are displayed. It is basically a handshake code, also the ECM self-tests the fan.

thats the problem, after following those procedures precisely the light doesnt flash but stays lit. The fan runs without anything connected to the ALDL. The ECM wont flash its codes.

Jumping the pins or having a scan tool connected to the ALDL doesnt change what the SES light does which is that it stays lit when the ignition is in the on position and doesnt flash at all.

You were not real clear about the sequence. If you are not getting a handshake code there is a problem with communication in the data bus. Did you try to reseat the prom? Assuming all else is OK(voltage, grounds) it is probably internal to the ECM. Try tapping on the ecm housing and see what happens. Many times they develop intermittent connections internally, unfortunately it is a coated board and tough to work on. Pull the prom cover off and try to tap on it also and see if anything changes.

My girlfriends car is doing the same thing...as soon as you start the car, the check engine light is on, but when you jump the terminals (ignition on, engine off) it just turns on the fan and the light doesn't come on at all. We figured the ECM was fried, so we tried another one (reman) and its doing the exact same thing. I didn't notice how well the PROM was seated the first time, but I know it was well seated the second time (replacement ECM). Could the PROM be bad maybe? I can't figure out any other possibility, since the light will come on when its running.

It took me a while to troubleshoot this but I figured it out, my original prom was bad so I had to find the program for my car (which I still havent found) , after a lengthy search and buying a prom burner I burned a chip with the closest bin program to my original. My car is an LB9 5spd with a 3.45 rear from California and this car has its own program specific to it.

The solution is in the prom, yours may be bad or is the wrong one. Order a new one thats correct for your car or burn one and it should fix the problem.


Good Luck

Thanks a lot, I was leaning in the direction of the PROM but its good to know that fixed your problem. We have a spare we're going to try tomorrow night, and hopefully it will do the trick. Thanks again!

Very interesting. Never came across a defective PROM, usually the ECM is the culprit. Did you use a static wrist strap when handling the PROM. Thats about the only time I see them fail. Glad you solved it.

The Prom had been in for over a year when the problem began. The spare Prom was the wrong #, so we ordered a new one today. Ive never had a Prom go bad either, but there doesnt seem to be any other logical explanation for it .

I've had where the ECM backup wire off the batt worked loose and then not be able to retrieve a code.

my cars doing the same exact thing..i know this thread is 8 years old but yeah..

http://www.thirdgen.org/techboard/tp...ll-when-4.html (motor wants to stall when at a stop?)

did it end up being the PROM?

I'm not sure as I read this above posts if this might be an issue, but the ECM has two different diagnostic modes. The first is the key on engine off (do not drive) "diagnostic mode". To enter this mode, you must have terminals A&B jumped when you turn the key on.

The second mode is "field service mode", where the service engine soon light will flash at different frequencies to indicate closed or open loop. To enter this mode, you jump terminals A&B while the engine is running. By jumping terminals A&B with the key already on, you may not be properly entering diag mode. Be sure to jump the terminals before you turn on the key.