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Q: Anti theft problem

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I think there is a short in the anti theft system. Will taking out the 7.5 anti theft fuse will that stop the alarm system?

My car has 177777 miles.
My car has an automatic transmission.

A: Rarely will pulling a fuse disable an anti-...

Rarely will pulling a fuse disable an anti-theft system. If it were that easy, thieves would figure that out quickly and more cars would be stolen. Fortunately, most factory alarms are usually fairly simple to diagnose but very difficult to bypass. By far, the most common failure are the door switches.

The door switches that turn on the interior lights and will light the red door light on your dash when you open a door or the door is ajar are the most common and easily overlooked failures. These same switches inform the BCM (body control module) when a door is opened without a key. If your door ajar light comes on while driving, this could be your problem. To test the switches, simply open doors one at a time and observe the door ajar light on the dash. If you open a door and the light doesn’t turn on, you have a problem in that door. Be sure to turn the key on and open doors one at a time and keep the rest closed. The trunk and engine hood will have switches as well.

Of course, check all fuses, not just the ones you think power the BCM. Often other fuses will affect other systems even though they are not labeled as such.

If this doesn’t help you figure out the problem, I personally would hook up a scanner and observe BCM data. The BCM is the primary module in the anti-theft system, but this car has door modules as well. They will monitor the lock position and window position of each door. If the data in the scanner doesn’t agree with the state of the door, you have found a problem.

Other common points of failure to consider are broken wires in the door jamb wiring harness. This happens because every time the door opens, this harness will flex eventually breaking a wire or wires in the wiring harnesses to the doors.

Last, there is a relay in this system that is in the engine compartment. Relays often develop bad connections internally. If this relay is cheap, I would just replace it as a test. If not, some testing will need to be done. To test the relay, you will need a wiring diagram and some knowledge of how automotive electrical systems work. If you want to have this checked for you, a technician from YourMechanic can come and diagnose your electrical system to determine how to proceed with a repair.

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