Rick's Free Auto Repair Advice

Security light on in GM vehicles — What it means

Learn what to do if you see the security light on in a GM vehicle.

Any time you see the security light on in a GM vehicle, that’s a sign of a problem with the immobilizer system. GM has made multiple changes to their immobilizer systems over the years. In some cases, the system will let you start the vehicle and drive, but in the vast majority, the system will disable the starter or fuel, or let the engine start and then immediately shut it off.

GM allows you to reset the immobilizer, allowing you to drive until you get the system fixed

At first, allowing a reset makes no sense because any car thief could simply perform it and drive off. But it’s not that easy. The reset procedure is intricate, requiring precise timing and requiring as long as 30 minutes in some cases. GM figures that car thieves aren’t going to sit in a vehicle for 30 minutes. If your security light is on and your vehicle won’t start, or starts and then stalls, try using the reset procedure using the links below.

GM made several different types of anti-theft systems. VATS, PassKey, and PassLock. The troubleshooting and reset procedures are different for each. PassLock seems to have the highest failure rate. If you have a key WITHOUT a chip in it, chances are it’s a PassLock system. This article describes how to troubleshoot and fix a PassLock system. But here are some more articles to help

Click here to see the reset procedure for PassLock

Click here to see the trouble codes for a PassLock system

Click here to see a typical PassLock wiring diagram

IF you have a PassKEY system, click here to see the reset procedure

If you swap out a PCM in a late ‘90’s GM vehicle and encounter an anti-theft light, you may not have to take it to the dealer for reprogramming. The PCM is set up to recognize a certain pattern as it relearns the key code. You can duplicate the pattern by performing this sequence exactly:

1) Hook up a battery charger and set at a low rate of charge.
2) Turn the key to crank the engine just long enough to engage the starter.
3) Leave the key in that position for at least 10 minutes. (Some shops say it takes as long as 30 mins).
4) When the theft light turns off, turn the key to OFF for 5 seconds and then back to start and leave key in that position for 10 minutes.
5) When the theft light turns off a second time, the PCM has learned the new key code and you can start the engine.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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