Pass Lock II is used on many GM vehicles from 1995-2005 and can be identified by a security light on the dash and by an ignition key with a “PK2” stamped on the metal part of the key where it meets the plastic end. There are some keys that may not have the “PK2” stamped on them and are also used for this system. These keys only function to turn a lock cylinder that houses a magnet in the center of the key cylinder assembly
(Figure 1) 
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When the lock cylinder is rotated in the crank mode it will travel far enough for the magnet to go past the sensing device allowing the sensor to send a signal to the security module.
The sensing device is a 3-wire Hall Effect sensor
(Figure 2) 
that the security module feeds a referance ground and referance power on 2 of the wires. The 3rd wire is a 5V referance signal line from the security module that the sensor will pull to a certain level when the magnet passes the sensing device. The hall sensors each have their own unique end result value between 0.5-4.5 volts when pulling the 5V ref low and this value will be learned by the security module and kept in its memory. When the battery goes dead or a new hall sensor is replaced you will need to relearn the Pass Lock sensor value into the security module.
The learn procedure is as follows:
Turn the lock cylinder all the way to the crank mode so the magnet on the lock cylinder can travel pass the Pass Lock sensor. Now let go of the key so it can return to the run position. The 5 Volt line will now be pulled to a value below 5 volts and the security light may begin to start flashing or just remain on solid. Some vehicles may start up and stall or may not crank at all. You must leave the ignition key on for 10 minutes and place a battery charger on the battery if needed in case the battery is low on charge. After 10 minutes you will see the security light go out or just stop flashing and then go out. At this point you turn the key off for 5 seconds. After 5 seconds you turn the lock cylinder to the crank position again to repeat the same process .You must perform this process 3 times for a total of 30 minutes of wait time. On your 4th attempt the vehicle should start up and continue to run.
This procedure does not apply if the vehicle has a Pass Lock III system. This system can be identified by a Security module & antenna straddling the lock cylinder. The keys for the Pass Lock III system are also different in nature. They will have a “PK3” or a “+O” stamped on them. These keys are true immobilizor keys and have tiny chips imbedded in them. The Pass Lock III systems require a differ learn procedure.
