Rick's Free Auto Repair Advice

GM PassKey and PassLock overview

Learn how the different GM PassKey and PassLock immobilizer systems work

GM has used PassKey and PassLock style immobilizers on their vehicles. When you get a no start, intermittent start, or start then die, you should first consider that you have an immobilizer issue. Determine which system you have in your vehicle, then diagnose the anti-theft system before you start replacing starting components. A tell-tale sign that you’ve got an anti-theft problem is a flashing “Theft” light. Here’s a brief primer on each system and what to look for.

Each GM anti-theft system disables different components. Some disable the engine starter, while others enable the starter but disable fuel injectors. None of them disable spark. So, first understand the difference between vehicle theft deterrent systems.

The differences between PassKey and PassLock

GM used two strategies to determine if someone was trying to steal the vehicle. The early systems used the first-generation PassKey system. Later, GM changed to the PassLock system. The PassKey systems verified authorized users by the key. The key construction varies over the years, with the early systems using a resistor pellet in the key and later systems using a transponder in the head of the key. See the changes in each generation listed below.

In the PassLock system, GM took the position that the only thing that mattered was if the key turned the lock cylinder. Their theory was, if the key turned the lock cylinder, then it must be an authorized user. To combat thieves who might rip out the lock cylinder, the lock cylinder case was designed to detect tampering. That’s why this system is referred to as PassLOCK.

PassKey III transponder key

PassKey III transponder keys are identified by the PK3 mark on the shank

PK2 PassKey and PassKey II

See the resistor pellet on the shaft of the key to the left of the plastic.

PassKey I

The first-generation PassKey systems incorporated a resistor pellet into the head of the key. The resistor made contact with terminals in the lock cylinder and the system read the resistance value of the resistor pellet. These systems had a high failure rate, so GM changed to the PassLock system.


PassLock systems are based on the theory that car thieves will use a slide hammer to yank the lock cylinder out of the housing and then try to turn the ignition switch with needle nose pliers.

The PassLock system works when the lock cylinder moves a magnet past a Hall effect sensor. The sensor then sends a signal to the control, which GM has moved several times over the model years. The control then sends the code to the PCM. If the code is correct, the PCM will allow the car to start and run. However, if the thief yanks the lock cylinder, it removes the magnet as well, so the sensor can’t detect the magnet’s movement. That disallows starting.

This system proved VERY unreliable as the Hall effect sensors had an extremely high failure rate. To repair the system, the shop would have to replace the lock cylinder housing because the Hall effect sensor can’t be replaced separately.  So GM moved to PassKey III

How PassLock and PassKey prevent vehicle theft

PassKey III Starts and Stalls if the Wrong Key is Used

In PassKey III, GM incorporated a transponder into the key head. As the driver moves the key toward the lock cylinder, a transmitter sends an interrogation signal to the transponder key. The transmitter is located in a ring surrounding the outer edge of the lock cylinder. The transponder receives the interrogation signal and responds with a coded message. The message is checked for proper response. If it’s correct, the vehicle will start and run. If not, the vehicle may start but will stall within seconds.

VAT Disables the starter if the wrong key is used

The VAT, or vehicle anti-theft was used primarily on 1986-88 Corvettes. The key had a pellet that was married to the vehicle anti-theft module at the factory. If the lock cylinder was turned or bypassed without the proper key installed, the system would disable the starter and kill the injectors. You could NOT perform a system “relearn.”

PassKey I, and II Starts then stalls if the wrong key is used

Similar to the VATS system, these vehicles also used a pellet key. You can identify this system by looking at the key. It has a black resistor pellet with a thin metal edge running along the key. The system enables the starter, but shuts down the injectors after initial startup. So the vehicle would start, then die. This system has a “relearn” feature.

PassKey III disables the fuel injectors if the wrong key is used
This system uses a special key (PK3 stamped on the shoulder of the key) with a transponder under the rubber cover. The receiver module is located next to the lock cylinder. When the key is positioned next to the lock cylinder receiver module, it energizes the transponder which sends a coded signal to the receiver. The receiver module checks its memory to see if the signal is correct. If it is, the module sends a signal to the PCM to enable the engine. If the value is valid, the indicator light will remain steady for 2 seconds. You cannot test this system with an ohmmeter. You need a scan tool. If the signal is not correct, the PCM disables the injector and the starter and flashes the light once per second. If an illegitimate key is used or a system problem exists, the light flashes twice per second.

Passlock starts then stalls
This system allows starter operation, then kills the injectors 1-2 seconds after startup. This is a “Hall-Effect” system. The key cylinder housing has a small magnet that rotates past the Passlock sensor. The sensor is powered and detects the rotation. The voltage on the sensor then drops to a specified value (there are 10 different values). There’s no pellet in the key. So basically this system is checking to see that a thief has not popped the lock cylinder to steal the vehicle. you have to perform a Powertrain Control Module (PCM) relearn procedure (need a programming scan tool).

DIAGNOSE PassLock and PassKey

PassKey I and II System Diagnostics

There are 15 different key codes. Measure the resistance of the key pellet by connecting an ohmmeter to each side of the key pellet. Once you find the resistance value, remove the lower dash panels and locate two small wires (both white) that run down the base of the steering column. Disconnect the electrical connector where those wires terminate. Then connect an ohmmeter to that electrical connector and insert the key. The reading you get at the end of the two white wires should match the resistance value you got from the key. Next, turn the key. The value on your meter should NOT change or drop out. If it does, you will have to replace the key cylinder. It is COMMON for the wires to break from the key cylinder.

If you have the PassKey system and the engine cranks, runs, and quits AND the THEFT light is on (for about 5 seconds), here are the tests you should perform:

1) Disconnect the electrical connector at the deterrent module. Then turn the ignition switch to RUN while you test the voltage between the dark blue wire in the theft deterrent electrical connector (the one you just disconnected) and a ground point. You should see 5 volts. If you don’t get that voltage, check for an open or short between the dark blue wire and the PCM that supplies the voltage to the module.

If you DO get the 5 volts, turn the ignition off and reconnect the electrical connector. Pierce the insulation on the same wire (with a pin) or back-probe the wire in the connector. Turn the ignition to RUN. The voltage should now read 2.5 volts. If not, replace the theft module. THAT assumes you have already conducted the key pellet test (explained above) and that it passes the test. If you see 2.5 volts and the vehicle still won’t stay running, check the fuel pump relay.

Here’s how it works, The lock cylinder reads the resistance of the pellet in the key and transmits that data to the theft module. Meanwhile, the PCM provides 5 volts on the dark blue wire. When the theft module determines that the proper key has been inserted, it partially grounds the voltage on the dark blue wire. The PCM initially provides fuel injector pulse, but if it doesn’t sees a voltage drop on that dark blue wire, it cuts off the fuel injectors. That’s why you get a crank, start, engine shut down.

So, if the vehicle starts and then dies, AND THE THEFT LIGHT FLASHES, the module is not getting the proper signal from the lock cylinder or the module is bad.


Testing this system is similar to the Passkey I and II systems. You want to check the voltage at the bottom of the steering column as you insert the key and rotate it check the yellow signal wire (should be 5 volts before you start the testing). Turn the key.

The value should drop. If it does not, you will have to replace the key cylinder. The “control module” for this system is built into the instrument panel control (IPC) in early years. In later years, the system is controlled by the Body Control Module (BCM), the Electronically Controlled Orifice-steering Assist (EVO), IPC and other systems. If you have to replace the lock cylinder or any of the control modules,

Passkey III

You cannot check this system with an ohmmeter. You will need a scan tool to watch the operation of the system and find the trouble code. Once you get the trouble code, follow the diagnostic procedure based on the specific code.

Some companies make PASSLOCK OVERRIDE SYSTEMS. I don’t have any experience with them.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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