Repair Topics > Alarms and AntiTheft Systems > How Passlock works

How Passlock works

GM’s Passlock antitheft system uses a normal ignition key. The anti-theft portion is located inside the lock cylinder case; that’s why it’s called Passlock instead of the other antitheft versions known as PassKey. Here’s how passlock works:

When you insert the proper ignition key and rotate the lock

Passlock wiring diagram

cylinder, a magnet mounted to the back side of the lock cylinder passes by a SECURITY hall effect style sensor mounted in the lock cylinder case. A hall effect sensor detects a magnetic field and sends a coded signal to the body control module (BCM). At the same time, the Passlock system will allow the starter relay to engage, which also allows engine cranking, ignition and fuel systems to operate. As the engine is starting, the BCM communicates the coded signal to the PCM. If the coded signal matches the previously programmed code in the PCM, the PCM will continue to allow the ignition and fuel systems to operate. If they don’t match, the PCM will disallow fuel and the engine will die. The BCM will light the SECURITY light.

Any key that can rotate the lock cylinder will cause the hall effect sensor to send a signal which will allow the vehicle to start and run. However, if a thief attempts to remove the lock cylinder with a slide hammer, a tamper sensor will detect the absence of the magnet and will immediately inhibit the start/run feature. In other words, thieves can’t steal these vehicles by popping the lock cylinder

They also can’t fool the system by placing a strong magnet on top of the steering column because that would automatically activate the tamper sensor. They can’t remove the SECURITY hall effect sensor and substitute a different part because the sensor signal is modified by one of ten different resistors. So thieves have only a one in ten chance of getting it right on the first try.

What goes wrong with Passlock?

Passlock is a failure prone system. In fact, owners can expect at least one failure in their ownership. The passlock SECURITY sensor or wiring harness have almost a 100% failure rate. The fix involves replacing the lock cylinder case and reprogramming the BCM and PCM. The SECURITY sensor is built into the lock cylinder case and can’t be purchased separately.

Is passlock causing the no start condition?

When the SECURITY warning light is on, a trouble code is stored in the computer. Get the trouble codes read with a scan tool that’s capable of reading Body “B” series codes. Here’s a list of the codes and their definitions.