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How Passlock works

Learn how Passlock works

Passlock works to prevent vehicle theft by shutting of the fuel injectors if a thief tries to remove the lock cylinder from the vehicle. Removing the lock cylinder with a slide hammer is a very common theft technique. Learn how Passlock works by reading this article.

Passlock components

• Ignition lock cylinder and key

passlock lock cylinder



• Ignition lock cylinder case, including the Passlock™ sensor

passlock lock cylinder case

Passlock lock cylinder case with sensor






• Ignition switch

passlock ignition switch

Ignition Switch






• Body control module (BCM)

body control module

Body Control Module





• Security indicator on the instrument cluster

• Powertrain control module (PCM)

How the GM Passlock works

Some vehicle security systems rely on a special transponder key that communicates with a receiver near the lock cylinder. The GM passlock system doesn’t use a transponder key. Instead, it relies on the correct key cut to turn the lock cylinder. If the correct key cut is used, the lock cylinder will rotate. But GM realizes that most car thieves used a slide hammer to remove the lock cylinder from the lock cylinder case and then use a screwdriver to start the vehicle. So the Passlock system concentrates on detecting the removal of the lock cylinder itself. The Hall effect sensor also incorporates a tamper circuit to detect the removal of the lock cylinder. The sensor can also detect the use of a different magnet.

To detect lock cylinder removal, GM incorporates a magnet on the end of the lock cylinder. They place a Hall effect sensor in the lock cylinder case. The Hall effect sensor changes its signal in the presence of a magnet. During normal operation with the correct key, the driver rotates the lock cylinder which causes the magnet to pass the Hall effect sensor. The sensor changes its signal and communicates with the body control module (BCM). The BCM communicates the signal digitally to the PCM for verification. If the signal is correct, it will enable vehicle start.

Lock cylinder case sensor and the BCM

Voltage to the lock cylinder case sensor is provided by the BCM. The BCM measures the voltage on the sensor circuit. The voltage value indicates whether the Passlock sensor has been activated by the rotation of the lock cylinder or whether the lock cylinder has been removed, indicating a tamper situation. The BCM checks the voltage and compares it to a programmed and learned value to see if it is valid. The voltage matches, the BCM sends a digital message containing a password to the PCM. If the voltage from the lock cylinder sensor is incorrect or the voltage indicates a tamper condition, the BCM will not send the correct password to the PCM and the vehicle won’t start.

How the PCM prevents starting

This part can be confusing because the PCM will always allow the engine to crank and start regardless of the condition of the Passlock system. In other words, the PCM will activate the starter relay and provide fuel injector operation while it completes the security verification process. However, if the PCM receives the wrong password from the BCM or no password, it will still allow engine cranking but will shut down the fuel injectors and stall the engine. The fuel injectors will remain off for 10-minutes even if the lock cylinder is turned from RUN to OFF and a restart is tried.

Security Light status

The security light turns on for 3 seconds whenever the key is turned to RUN or START.

The security light flashes if a trouble code is stored in the system.

Passlock wiring diagram

passlock wiring diagram

Passlock wiring diagram

Fixing a broken Passlock sensor

The sensor in the lock cylinder case is the most common failure point, along with wiring harness issues between the lock cylinder case and the connector at the bottom of the steering column.

Programming the new passlock lock cylinder sensor

You can replace the lock cylinder case yourself and perform a 30-minute relearn procedure without the use of a scan tool.

30 Minute Passlock Learn Procedure


Turn ON the ignition, with the engine OFF.

Attempt to start the engine, then release the key to ON. The vehicle will not start.

Observe the SECURITY light. After approximately 10 minutes the light will  will turn OFF.

Turn OFF the ignition and wait 5 seconds.

Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes.

The vehicle is now ready to relearn the Passlock™ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK.

Important: The vehicle learns the Passlock™ Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.

Start the engine. The vehicle has now learned the Passlock™ Sensor Data Code and/or password.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat

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