GM Passlock Anti-Theft Systems

The Passlock I system is considered the replacement for the GM Vehicle Anti-Theft System (VATS). The Passlock II and III (PK3) systems followed shortly after the introduction of the Passlock I system. The Passlock family of anti-theft systems are still in use today.

Reprogramming Passlock with a learning procedure

The Passlock family has two basic relearn procedures - a ten-minute procedure and a thirty-minute procedure - and which one you use depends on the system and what components have been replaced.

10-minute learn procedure:

  1. Start from the ignition in the OFF position
  2. Try to start the engine. It should crank but not start, and the security light will be flashing.
  3. Release the key to the RUN position and leave it there for 10 minutes.
  4. Turn the key to the OFF position for 20 seconds
  5. Try to start the engine again. If it starts and runs, everything worked!
  6. If the engine still won't start, repeat the above steps 2 more times (30 minutes total)

After the learn procedure is completed and the engine is running, the security light may stay on steady for a few minutes and then go off - this is normal. Depending on the system, replacement of key components will necessitate that a relearn procedure be completed. These include the ignition lock cylinder, Powertrain Control Module (PCM), Body Control Module (BCM), or Instrument Panel Cluster (IPC).

What does the warning light mean?

Operating states of the warning light on Passlock systems should be:

  • On for bulb check
  • Off when engine is running
  • On steady if a fault occurs
  • Flashing in the learn mode

Under normal operation, you should see the warning light on during bulb check and off when the engine is running—no message displayed on the DIC.

When the system detects a fault, the warning light will illuminate or a message will be displayed on the DIC, depending on the model. If a fault occurs while driving, the engine may or may not start after turning the ignition off. If a fault occurs when trying to start the vehicle, it probably won't start.

History of the GM Passlock family of anti-theft systems

In the mid 1990s, the first Passlock systems were introduced and are still used to this day. The Passlock I and Passlock II systems use a sensor in the ignition lock cylinder, which looks for proper rotation of the lock cylinder with the correct key. Upon successful rotation of the ignition lock cylinder, a code will be sent to the Passlock module.

The Passlock III system uses a transponder located under the plastic covering of the key. In most cases, these keys are stamped "PK3" on the metal blade near the plastic covering. An antenna located at the entry point of the ignition lock cylinder reads the transponder data from the key and sends a code to the Passlock control module. This system is very similar to the immobilizer system used by many other manufactures. Some higher end models use the key fob for the Passlock code and the starter will not turn unless a correctly programed key fob is present.

The Passlock III antenna and the Passlock I and Passlock II sensors in the lock cylinder are all commonly referred to as Passlock sensors. Upon receipt of the code from the sensor, the Passlock module will match the code received to previously learned values. If there is a match, the Passlock module will send a signal to the engine control module to allow the engine to start and run. The Passlock module is commonly part of the Body Control Module (BCM) or Instrument Panel Cluster (IPC).

2 User Comments

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By , December 27, 2010
I have a Saturn Ion 2003. Every winter I have to deal with this problem and from what I hear a lot of other Saturn owners also have this issue. Also that the '03 ion is known for this problem so why isn't it coverd as recalled defect?? I would really like to know this because every auto place I have taken it to has said there is nothing that they can do.
By , February 01, 2018
All right I've done the passlock procedure and it works every time the first try but the problem is is whenever it wants to it kind of like forgets Mikey and I have to do the relearning process all over again is there a reason for this that you might no of or a trick to maybe figuring out how not to do that or how it won't forget it I don't know I mean I'd appreciate it if you let me know thank you have a great day oh by the way it's a 1998 Chevy Tahoe LTZ Z71 with the 5.7 L motor and keeping mine now I just replaced the key and tumbler in the ignition this is the problem I've had ever since thank you

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