Depending on the make and model, there are various factory and aftermarket anti-theft systems. In this article, we are concentrating on factory-installed GM starter interrupt systems. This type of anti-theft system is designed to prevent the vehicle from starting unless the correct ignition key is used.
These systems used a dash-mounted "security" or "theft" light which can be yellow or red in color. Depending on the current status of the system, the warning light will be off, on steady, or flashing.
Although they may share the same dash-mounted warning light, these "starter interrupt" anti-theft systems work separately from and independently of the "content theft" systems available from factory and aftermarket suppliers. Content theft referrers to anti-theft systems that sound an alarm when a vehicle is broken into.
Two Types of Systems
In the mid-1990s, the first Passlock systems were introduced and are still used to this day. The Passlock I and Passlock II systems use a sensor in the ignition lock cylinder, which looks for proper rotation of the lock cylinder with the correct key. Upon successful rotation of the ignition lock cylinder, a code will be sent to the Passlock module.
The Passlock III system uses a transponder located under the plastic covering of the key. In most cases, these keys are stamped "PK3" on the metal blade near the plastic covering. An antenna located at the entry point of the ignition lock cylinder reads the transponder data from the key and sends a code to the Passlock control module. This system is very similar to the immobilizer system used by many other manufactures.
The Passlock III antenna and the Passlock I and Passlock II sensors in the lock cylinder are all commonly referred to as Passlock sensors. Upon receipt of the code from the sensor, the Passlock module will match the code received to previously learned values. If there is a match, the Passlock module will send a signal to the engine control module to allow the engine to start and run. The Passlock module is commonly part of the Body Control Module (BCM) or Instrument Panel Cluster (IPC).
In the mid-1980s, we first began to see the "VATS" (Vehicle Anti-Theft System). It is easily identifiable by the "resistor" mounted in the metal blade of the ignition key. There are fifteen different resistor values available. In order for the engine to start, the correct "resistor" must be read by the VATS control module via the ignition lock cylinder. Also, the ignition lock cylinder must be rotated to the "crank" position. If the incorrect code is read, the system will not allow the engine to start for four minutes—even if the correct code is received. The security light will remain on or a message will be displayed on the Driver Information Center (depending on the model) when this occurs.
Both of these starter interrupt systems are very effective on their own. If you install an aftermarket alarm system, be advised that the starter interrupt feature can cause problems with the factory-installed systems. If you are considering installing an aftermarket alarm system on a vehicle equipped with a factory-installed starter interrupt, you should consider not installing the interrupt option.
Symptoms and Solutions
- Passlock system fault code B2960
- Security light flashing
- Engine starts and dies
If you find yourself stranded, your engine won't start, and the security light is flashing, wait ten minutes. The flashing light should go off or illuminate without flashing. When it does, turn the ignition off, wait twenty seconds, and then try to start the engine. If it starts and runs, you should be able to make it home. Please remember that this is only a temporary fix; the fault will happen again. The most common repair is to replace the ignition lock cylinder or Passlock sensor and perform a relearn procedure.
- VATS (Vehicle Anti‐Theft System)
- Security light on steady
- Engine won't crank—all other systems functioning normally
If you find yourself stranded and the engine won't crank over, turn the ignition off and wait four minutes. Then, try again. If the engine starts, you should be able to make it home. Please remember that this is only a temporary fix; the fault will happen again. The most common repair is to replace the ignition lock cylinder due to broken wires inside the steering column.
- The Passlock I system was introduced in mid 1990's
- Passlock II and Passlock III (PK3 )systems were introduced shortly after
- VATS system were introduced in mid 1980's
What Does the Warning Light Mean?
Operating states of the warning light on Passlock systems should be:
- On for bulb check
- Off when engine is running
- On steady if a fault occurs
- Flashing in the learn mode
Under normal operation, you should see the warning light on during bulb check and off when the engine is running—no message displayed on the DIC.
When the system detects a fault, the warning light will illuminate or a message will be displayed on the DIC, depending on the model. If a fault occurs while driving, the engine may or may not start after turning the ignition off. If a fault occurs when trying to start the vehicle, it probably won't start.
The warning light on the VATS system is similar except it does not have the learn mode.
Most common problems related the the Passlock family involve the Passlock sensor. The sensor is part of the ignition lock cylinder on Passlock I and Passlock II systems. The antenna is the Passlock sensor on Passlock III systems. If these sensors fail, the proper repair would be to replace the failed part and perform a theft system relearn procedure.
By far, the most common problem with the VATS system is broken wires in the steering column. These wires connect the ignition lock cylinder to the under-dash wire harness. The proper repair is to replace the ignition lock cylinder, which comes with wires already attached that snake down the steering column. The new lock cylinder will come with a key blank that will only rotate the new lock cylinder—it does not have a resistor in it that will allow the engine to start and run. A new key must be made from the blank because it has the proper resistor value. Normally, the resistor is read from the old key using a tool called an "interrogator." The interrogator is available in the parts department of most GM dealers.
The Passlock family has two basic relearn procedures—a ten-minute procedure and a thirty-minute procedure—and which one you use depends on the system and what components have been replaced.
The ten-minute procedure involves trying to start the engine. If nothing happens or the engine starts and dies, the security light will be flashing. Wait ten minutes. The flashing light should go off or illuminate without flashing. When it does, turn the ignition off, wait twenty seconds, and then try to start the engine. If it starts and runs, everything is okay. If you get the same result as previously—nothing happens or the engine starts and dies with the security light flashing—wait ten more minutes.
You will now begin the thirty-minute relearn procedure, which is just the ten-minute procedure repeated three times. After turning the ignition off for the third time and waiting for twenty seconds, the engine should start and run. If it doesn't, there may still be something wrong with the system.
After the learn procedure is completed and the engine is running, the security light may stay on steady for a few minutes and then go off—this is normal. Depending on the system, replacement of key components will necessitate that a relearn procedure be completed. These include the ignition lock cylinder, Powertrain Control Module (PCM), Body Control Module (BCM), or Instrument Panel Cluster (IPC).
Unless the VATS control module is replaced, there is no relearn procedure for the VATS system. A new VATS module will learn the resistor code the first time it is powered up. Once the module learns its code, there is no way to relearn it. New ignition keys must always be matched to the learned code in the VATS control module. If all keys are lost and the VATS code is unknown, then use trial and error to find the correct VATS code. (In some cases, the VATS code may be available from your local General Motors parts department.) If the incorrect VATS code is read by the control module, the system will not allow the engine to start for four minutes—even if the correct code is received.