03 Chevy Impala PCM Re Learn Problems

Hello I need to change my PCM on my 03 Chevy Impala so I purchased a used one. Has the same numbers as my original. I installed it and tried following the Re learn steps and could never proceed to step 2 because the security light never stopped flashing. I waited over the 10 minutes time frame and it was still flashing. Actually the security light was flashing and so was the battery light. Does this PCM need to be flashed to my VIN? Not sure what steps to take next.

When swapping out the PCM, the broadcast code and the production PCM number must be transferred to the replacement PCM.

I hope you still have the old PCM.


I do. Will have to find someone to do that.
Thanks for your answer

May I ask how you determined that you needed to replace the pcm . . . ?!

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My Check engine kept coming on. It was throwing the code PO481
Cooling Fan 2 Control Circuit Malfunction. Changed Fans and changed Relays. Changed coolant temp sensor. Still throwing the code. I clear it out and the check engine comes back. So I decided it was a fault in the PCM

I am going to bet when you get this PCM coded to your car it will still throw the P0418. If I look up the causes, a bad PCM is not one of them… these are;

  • Faulty fan control relay
  • Faulty cooling fan 2
  • Fan control relay harness is open or shorted
  • Fan control relay circuit poor electrical connection
  • Fan cooling fan harness is open or shorted
  • Fan cooling fan circuit poor electrical connection

You covered the 1st 2, but not the bottom 4. Or at least you didn’t tell us you checked the harness for shorts, opens or poor connections. I hope I’m wrong and the PCM fixes it.


I think you’re onto something. And I’d be very surprised if the problem was with the PCM.

I own an Impala (3800 LS) from the same “generation.” I’ve got the factory service manual, with the car, but I’m 1,500 miles away from them right now.

However, if I recall (good memory… just really short), there was a technical service bulletin or two pertaining to something (I’m straining to recall and picture the engine compartment) chafing the under-hood wiring harness, I believe where it goes just under/around a body element/component on the passenger’s side front of the engine bay. (Brain fart…Is the battery tray over there?) I’d look there or search the (TSBs) bulletins.

Also, GM has bulletins pertaining to “fretting corrosion” and advises unplugging and re-plugging harness connectors to establish better contact, I’d say especially on an older unit, such as this one.
:palm_tree: :sunglasses: :palm_tree:

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Thanks everyone. After reading these posts I decided to go back and re check some things. Started with checking the Fuses for the Fan control 1 and the Fan Control 2-3. They are 25 amp. Used my Volt Meter to check each side of the fuse and found the fuse for Fan Control 2-3 was blown on one side. It shows 12 on one side and nothing on the other. Changed that fuse. Took the car out ran some errands and no Check Engine light so far. This is the longest I have gone without the Check Engine light coming on.

I’m hoping this solves the problem.
Still wondering why that fuse would have gone. And if it goes bad again where would I look to find what is causing it.

Cooling fans are a high current draw item. It could be the fuse just gave up due to age and the continued application of high current. The current draw gets even higher as the fan motor ages and wears

The current draw of the fan when new is unknown to me. Comparison of that number compared to the old existing fan motor would likely show a significant increase when checked with an ammeter. Buy a new motor, check the current draw, and then compare it to the draw of the old motor.

This kind of problem exists with any make and model of car and is not confined to cooling fan motors either.
Some years ago my wife came in and said the cabin fan motor quit in our Lincoln so no cold air.
When checked it was working, blowing plenty of air, and quiet as could be. An ammeter check showed that fan was pulling 27 amps of current on the HIGH setting. That’s a godawful lot and could be looked at as a potential fire hazard. New motor pulled about 9 amps and cured the random quitting.

There’s a long list of electrical problems over the years with many other makes and models involving high current draw.

Why wasn’t that the first thing you checked before replacing fans and relays? Not trying to be be a smart-alec, but that is usually electrical diagnostic step 1.

Now one of those old fans may have been drawing much more current than they should and that blew the fuse. You replaced the motor but not the fuse, until now.

Been chasing this problem a long time. Initially we checked all the Relays and all the fuses. They all checked out fine. But then tested the fans and one was not working so I replaced it. Check engine light disappeared. Then a few months later the check engine light appeared again with same code. Winter hit and I just lived with the light. Spring is here so I went back at it and was give the suggestion that I should change the PCM. Well that did not go to well so I retraced my steps and found the bad fuse.
Hoping now the fuse does not blow again