One of General Motor's solutions for stopping thieves from driving away with a car is, coincidently, doing the same thing to the actual owner.
Passlock is an immobilizer introduced by GM in many mid-to-late 1990’s vehicles. Immobilizers are “electronic security device fitted to an automobile that prevents the engine from running unless the correct key (or token) is present.”1 Well, Passlock should be called the Great Immobilizer because not only does it stop thieves with no keys, it also stops owners with the right keys.
Passlock uses a coded lock cylinder that essentially stops the engine from running until the proper key is detected. When the key is inserted into the ignition, a magnet on the cylinder creates a signal to the ECM (engine control module) that essentially says everything is ok to start and run.
Passlock Light On Preventing Engine to Start
The short answer is there’s a problem with the sensor. Some have suggested that the pins in the system are “tin” plated and subject to oxidization over time. Others say their system is possessed by the devil and in need of an exorcist. I’ll let you decide which one sounds more accurate.
When the sensor defect starts, it usually presents itself as system fault code B2960. A red (or yellow) “security” warning light will stay illuminated and the engine will either a) never start or b) start for a moment and then shut down.
A Temporary Fix
A temporary fix is to leave your key in the ignition, at the “auxiliary” position for about 10-15 minutes until the security light shuts off. When the light shuts off you can try to start the engine again. Rinse, repeat, and cry.
A more permanent solution is to either replace the ignition lock cylinder or bypass the sensor.
Replacement cylinder with new sensors can cost, with labor, as much as $450.
Bypassing the sensor will void any warranty, but since most of these vehicles are out-of-warranty at this point that might not be of any concern. NewRockies.com has a solution for how to fully bypass the PassLock system in GM cars.