Anti-Theft Systems for GM Vehicles
GM has used a variety of anti-theft systems on their vehicles. When you get a no start, intermittent start, or start then die, you should diagnose the anti-theft system before you start replacing starting components. A tell-tale sign that you’ve got an anti-theft problem is a flashing “Theft” light. Here’s a brief primer on each system and what to look for.
Each GM anti-theft system disables different components. Some disable the engine starter, while others enable the starter but disable fuel injectors. None of them disable spark. So, first understand the difference between vehicle theft deterrent systems.
VAT
The VAT, or vehicle anti-theft was used primarily on 1986-88 Corvettes. The key had a pellet that was married to the vehicle anti-theft module at the factory. If the lock cylinder was turned or bypassed with the proper key installed, the system would disable the starter and kill the injectors. You could NOT perform a system “relearn.”
PassKey I, and II
Similar to the VATS system, these vehicles also used a pellet key. You can identify this system by looking at the key. It has a black resistor pellet with a thin metal edge running along the key. The system enables the starter, but shuts down the injectors after initial startup. So the vehicle would start, then die. This system has a “relearn” feature. It was used on these vehicles
Buick
Century 97-01
Park Avenue 91-96
LeSabre 92-99
Riviera/Reatta 90-01
Regal 94-99
Roadmaster 94-96
Cadillac
Eldorado 89-01
Seville 89-97
Fleetwood 92-96
Deville 90-99
Allante 89-93
Chevrolet
Camero 89-01
Corvette 89-01
Caprice/Impala 94-96
Lumina/Monte Carlo 95-97
Oldsmobile
Cutlass Supreme 94-97
Aurora 95-00
Regency/88 91-99
Toronado 90-93
Pontiac
Firebird 89-01
Grand Prix 94-96
Bonneville 92-99
PassKey III
This system uses a special key (PK3 stamped on the shoulder of the key) with a transponder under the rubber cover. The receiver module is located next to the lock cylinder. When the key is positioned next to the lock cylinder receiver module, it energizes the transponder which sends a coded signal to the receiver. The receiver module checks its memory to see if the signal is correct. If it is, the module sends a signal to the PCM to enable the engine. If the value is valid, the indicator light will remain steady for 2 seconds. You cannot test this system with an ohmmeter. You need a scan tool. If the signal is not correct, the PCM disables the injector and the starter and flashes the light once per second. If an illegitimate key is used or a system problem exists, the light flashes twice per second.
The system is used on:
Buick
Lesabre 2000-on
Park Avenue 97-on
Cadillac
Seville 98-on
Deville 2000-on
Oldsmobile
Aurora 01-on
Pontiac
Bonneville 2000-on
GM Minivans
Venture, Montana, Silhouette
Passlock
This system allows starter operation, then kills the injectors 1-2 seconds after startup. This is a “Hall-Effect” system. The key cylinder housing has a small magnet that rotates past the Passlock sensor. The sensor is powered and detects the rotation. The voltage on the sensor then drops to a specified value (there are 10 different values). There’s no pellet in the key. So basically this system is checking to see that a thief has not popped the lock cylinder to steal the vehicle. you have to perform a Powertrain Control Module (PCM) relearn procedure (need a programming scan tool). The Passlock system was used on:
Buick
Skylark 96-98
Chevrolet
Cavalier 95-on
Impala 2000-on
Malibu 97-on
Monte Carlo 2000-on
Oldsmobile
Achieva 96-on
Cutlass 97-on
Intrigue 98-on
All GM trucks 98-0n
SYSTEM DIAGNOSTICS
PassKey I and II System Diagnostics
There are 15 different key codes. Measure the resistance of the key pellet by connecting an ohmmeter to each side of the key pellet. Once you find the resistance value, remove the lower dash panels and locate two small wires (both white) that run down the base of the steering column. Disconnect the electrical connector where those wires terminate. Then connect an ohmmeter to that electrical connector and insert the key. The reading you get at the end of the two white wires should match the resistance value you got from the key. Next, turn the key. The value on your meter should NOT change or drop out. If it does, you will have to replace the key cylinder. It is COMMON for the wires to break from the key cylinder.
If you have the PassKey system and the engine cranks, runs, and quits AND the THEFT light is on (for about 5 seconds), here are the tests you should perform:
1) Disconnect the electrical connector at the deterrent module. Then turn the ignition switch to RUN while you test the voltage between the dark blue wire in the theft deterrent electrical connector (the one you just disconnected) and a ground point. You should see 5 volts. If you don’t get that voltage, check for an open or short between the dark blue wire and the PCM that supplies the voltage to the module.
If you DO get the 5 volts, turn the ignition off and reconnect the electrical connector. Pierce the insulation on the same wire (with a pin) or back-probe the wire in the connector. Turn the ignition to RUN. The voltage should now read 2.5 volts. If not, replace the theft module. THAT assumes you have already conducted the key pellet test (explained above) and that it passes the test. If you see 2.5 volts and the vehicle still won’t stay running, check the fuel pump relay.
Here’s how it works, The lock cylinder reads the resistance of the pellet in the key and transmits that data to the theft module. Meanwhile, the PCM provides 5 volts on the dark blue wire. When the theft module determines that the proper key has been inserted, it partially grounds the voltage on the dark blue wire. The PCM initially provides fuel injector pulse, but if it doesn’t sees a voltage drop on that dark blue wire, it cuts off the fuel injectors. That’s why you get a crank, start, engine shut down.
So, if the vehicle starts and then dies, AND THE THEFT LIGHT FLASHES, the module is not getting the proper signal from the lock cylinder or the module is bad.
Passlock
Testing this system is similar to the Passkey I and II systems. You want to check the voltage at the bottom of the steering column as you insert the key and rotate it check the yellow signal wire (should be 5 volts before you start the testing). Turn the key.
The value should drop. If it does not, you will have to replace the key cylinder. The “control module” for this system is built into the instrument panel control (IPC) in early years. In later years, the system is controlled by the Body Control Module (BCM), the Electronically Controlled Orifice-steering Assist (EVO), IPC and other systems. If you have to replace the lock cylinder or any of the control modules,
Passkey III
You cannot check this system with an ohmmeter. You will need a scan tool to watch the operation of the system and find the trouble code. Once you get the trouble code, follow the diagnostic procedure based on the specific code.
For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair.
If you just need information for a single repair and want to save money,eautorepair offers a lower price 1-week subscription for only $11.99. Or, if you’ll be working on this vehicle in the future, you can buy a 1-year subscription (Alldatadiy.com for $26.99, or eautorepair.net $29.99)
© 2007 Rick Muscoplat
Alldatadiy.com
eautorepair.net
Both Alldata and Eautorepair offer professional grade shop manuals at a price you can afford. Check them out!.
Hover your mouse over the REPAIR ARTICLES tab at the top of the page. The click on DIAGNOSTICS to search by SYMPTOM, TROUBLE CODE, or read TECH TIPS on how to diagnose a problem
Click on MAKE an MODEL to see all the articles I’ve written for your vehicle
Click on REPAIR TOPICS to read individual articles written for specific automotive systems
If you’ve had your car scanned and know the trouble code go to TROUBLE CODES
Here are some quick links to speed up your research
Click on the link below for speedy navigation:
ABS issues, how to bleed ABS brakes
Alternator issues or how to test
Belt diagram or serpentine belt issues
Brakes, brake issues, what kind of brake parts to buy, ABS issues
Car Alarms or Antitheft systems
Cooling system, Water pump, What type of coolant to use
Firing order, cylinder layout, finding #1 cylinder
Oxygen sensors, How they work, How to change, How to test
Window issues, power window motor, replace window regulator, glass adhesive
Hi, this is great info. Quick question, I have recently replaced the key lock cylinder on my 2004 chevy malibu classic, and did the vats bypass (leave ignition in “on” position for 10 min, turn key to off, then start car). The car seams to have learned the new cylinder system and runs fine, but I’ve read this is a temp fix. Anybody know any specifics on this?
It’s a temp fix if you DON’T replace the lock cylinder. If you do, it should be a perm fix (permanent in GM language, means until the new cylinder breaks)
where is the antitheft module located on a 1995 bonnevile
I have a 1998 Deville,just replaced the digital speedo cluster (previous one burnt up), I get message starting disabled.Is the anti-theft in the cluster?Can I bypass the vats?I’m assuming my key doesn’t have the proper resistance to match the cluster. any help?
where is the antitheft module located on a 1998 Cavalier
In this vehicle, the anti-theft module is built into the instrument control panel. The anti-theft sensor communicates with the instrument cluster and the cluster communicates with the powertrain control module. The PCM is located behind the dash on the passenger side. Read the advice under Repair Topics -> Alarms and Anti-theft before replacing the instrument cluster. Perform the tests outlined there. If you determine that the cluster is bad, there are several companies that can rebuild them rather than pay for a new one.
96 achieva bought new.
I had this cluster replaced years (2004) ago and it worked fine. The car has been parked for a few years (since 2007) and I just gave it to a mechanic friend. We started it just fine with a jump start and the cluster was shot again but It ran fine !
He bought a used working clusted and put it in and the cluster works, but it dies right away with the passcode injector lockout function. Could it be that the cluster has a different code than the key?
Is the sensor that detects the magnet rotation bad?
Please help.
Did you perform the relearn procedure?
Greetings,
I have a 97 Olds Regency that is experiencing(I guess that means I’m experiencing) a no start problem. No engagement of the starter solenoid, but the starter enable relay is working. There’s no voltage at the purple wire on the starter solenoid. Security light is flashing. Any suggestions would help. Tried two different keys, no luck. Thanks, Franz
I was told that on a ‘95 Caprice/Impala Police Package that there is not a resistor in the key????? Is this true??? I am having the start,,run,,cut off problem. Any advice on old Police cars would be well welcomed.
Your vehilce has the PassKey system where a resistor pellet is embedded in the key. Scan this article and perform the tests shown in it to diagnose your problem
PassKey Diagnostics
I cannot tell you if the police version has an anti-theft system. However, the ‘95 Carpice/Impala used the PassKey system with a pellet in the key. The diagnostic procedure for the “crank but no start–with theft light lit for 5 seconds” is: 1) Disconnect the electrical connector at the deterrent module. Then turn the ignition switch to RUN while you test the voltage between the dark blue wire in the theft deterrent electrical connector (the one you just disconnected) and a ground point. You should see 5 volts. If you don’t get that voltage, check for an open or short between the dark blue wire and the PCM that supplies the voltage to the module.
If you DO get the 5 volts, turn the ignition off and reconnect the electrical connector. Pierce the insulation on the same wire (with a pin) or backprobe the wire in the connector. Turn the ignition to RUN. The voltage should now read 2.5 volts. If not, replace the theft module. If you see 2.5 volts and the vehicle still won’t stay running, check the fuel pump relay.
Here’s how it works, The lock cylinder reads the resistance of the pellet in the key and transmits that data to the theft module. Meanwhile, the PCM provides 5 volts on the dark blue wire. When the theft module determines that the proper key has been inserted, it partially ground the voltage on the dark blue wire. When the PCM sees voltage drop on that wire, it enables the fuel injectors to provide fuel.
However, if the vehicle starts and then dies, AND THE THEFT LIGHT FLASHES, the module is not getting the proper signal from the lock cylinder or the module is bad.
Hi. This may be a long shot, but i’m lost. I recently had to replaced the fuel pump in my 1999 sunfire (2.2L 5 speed) – after burning the old one up (no gas). The new pump engages correctly, but the car will not start. I tried several times, and pulled a plug to see if it flooded – they were dry. The anti-theft light comes on when the key is ahead (so does all other, but don’t go out). I assume the injectors have shut down. It is my understanding that there is a key sequence to reset the pass key system (theft). Any help would be greatly appreciated. Thanks
You can reset the PassLock system if you’re replace the faulty component. But if you haven’t fixed anything, the reset procedure won’t solve anything.
Rick,
I recently received a 2000 Chevy Malibu 4 door with the 3.1 liter engine about a month ago. It had about 137,000 miles on it and ran just fine. They last Sunday Dec. 13, 2009 it failed to start. Because I’m on disability despite my knowledge and having worked on my own cars for year, I don’t have a lot of money to take it into the dealership for repair. So I’ve been trying to learn if this is a relatively simple problem that I can fix myself. I have read quite a bit about this Passlock system and was told by the local Chevy mechanic that is the most likely cause of my problem. It fits with what you wrote in your article. My problem is that I don’t want to be bothered by this anti-theft system period. It’s more trouble than it’s worth for this older vehicle. I’ve read a few places that this system can be bypassed. Is that true and can you tell me how to do it. If I knew where the wire to the fuel pump is located, I could disconnect it and put in my own power switch to the fuel pump, but I don’t know where this is. Any help would be much appreciated. I’m with out wheels at this time because of this.
You can install an immobilizer bypass system (used when you install a remote start). But that will cost more that the PassLock repair. Those are the only options. There is no “backyard” hack to disable this system. It won’t help to hotwire the fuel pump since the computer is actually disabling the fuel injectors themselves. If they won’t fire, it won’t start.
i have a 1998 sunfire 2.4 .I put a 2000 2.2 engine and tranny(the whole cradle),never unhooked any wires to the engine.I just pluged the 2.2 up to the computer and it did start but it cut off after a couple second.While i was trying to start the car i realized i had the bcm module unhooked,plugged it back up & still no start.Tried switching the ign. switch & the computer to the 2.2 & still nothing.I put everything back original and hooked up a pljx …..nothing.I read the pljx only works if u have a rpm signal.My ? is ,what if i hook up a fuel injector cleaning system?I would then have the engine running but the injector signal would still b dead.Also there is a jaycorptech website that shows relays and resistors .Any suggetions would help
Start with the basics. See if the crankshaft and camshaft sensors are operating properly. Then check for spark. Check to see if the fuel injectors are firing. The computer is not going to command fire or spark without good crankshaft or camshaft signals.
i have a 96 cadillac deville. i need to bypass the radio anti theft, but i dont know how. I cant open my gas door or my truck. help
I’m not aware that the radio has anything to do with the gas door or trunk. My understanding is that the remote keyless entry module is built into the instrument cluster. I could be wrong. Consult a factory manual to be sure. I have NO idea how to bypass this. Not sure you can.
1995 olds eighty-eight, removed remote start, car will not crank, checked ignition resistance with key in, same as ohming pelet in key, fuel pump comes on, but no power at puple wire at starter sol, tried relearn procedure did not work. car drove into my garage but now will not crank any suggestions would be apprieciated
Ok, so you checked the resistance with the key in. But did you check the resistance value as you rotate the key to the start position? I bet you’ll see the value change or drop out.
I just had the antitheft module replaced by a dealer on a 98 Chevy pickup. Truck starts now, but the security light remains on, and the dealer says that its due to a prolbem with the Passlock system–and I now need to replace this ($350 or so), or problem with antitheft module could happen again. I’ve seen bypass systems online for $50-75–can I simply bypass the system?
I’ve never used a bypass module so can’t offer any advice.
I have a ‘97 Cutlass Supreme and have replaced the entire steering column to repair a bad multifunction switch (no signal lights). The signal lights work fine now, but I have a no crank condition. If I apply power to the starter solenoid the car starts and stalls a few seconds later. My question is does an improper voltage drop in the Passlock system cause a no crank condition or just shut down the injectors after a few seconds? If so, am I right in thinking that performing a ‘VATS’ bypass as per Marco’s comment (June 2/’09) should cure this problem?
You don’t have PassLock. You have PassKey. PassKey DOES cut out the starter circuit and the fuel system. Rather than bypass it, why don’t you remove the lock cylinder and transponder from the old column and transfer it over. I’m not the person to ask about the VATS bypass. Never installed one and never would.
how can i bypass or repair the anti theft device on a 1995 monte carlo
To repair the anti-theft, read the article on my site and test and replace the components. I know that a few companies make a bypass kit. I’ve never installed one so I can’t give advice on how to do it.
Is there a way to just remove an anti theft on a 95 monte carlo?
Nope. Either fix it or find someone who can install a bypass system. No way to remove it. P.S., by the time you end up screwing around with a bypass, you could have fixed the system.
I just replaced my ign. switch on my 2000 saturn sl2 (the cyl. locked up) it starts but dies after a couple of seconds, the security light keeps flahing on the dash (I think I tripped the theft deterant system) do I have to call the dealer for a re-activation code? and can I do this with an obd2 ?
You do not need to go to the dealer. And no, you cannot reset it with a code reader (you need a bi-directional scan tool). But you can do the relearn procedure yourself. Turn the IGN switch to RUN. Momentarily turn it to START–BUT DO NOT START THE VEHICLE. Wait 10 minutes–the security light will flash. When the security lights stays on steady turn the IGN switch to OFF. Do this procedure 3 times. This will take you 30 minutes. On the 4th try, start the vehicle. Oh, bring a book or magazine along.