1991 Mercury Topaz Antitheft System Diagnosis & Testing Cost

Most vehicles are factory equipped with some sort of technology to prevent or deter theft of the vehicle and it's contents. These systems get the generic name of anti theft systems, but there are great differences between systems depending on year make and model. For instance, older vehicles have used keys with embedded resistors, and many modern vehicles use transistors and infrared sensors on keys to prevent unauthorized starting of the engine. But the systems are much more intricate than just a secure key.

Anti Theft systems work by adding components to the vehicle to detect the proper key at every position of use, and making the system redundant to avoid bypassing security systems. Most modern vehicles first have a wireless control module which must accept the key fob before unlocking, locking, remote starting, starting with the key, or starting the vehicle with the push button start feature. This module will interact with the key at various locations in the vehicle and outside of the vehicle when within a certain distance. To prevent tricking the wireless control module, the module must be checked by one or more other control modules, likely an anti theft system control module, body control module, and engine control module, before it can allow the doors to be opened or the engine to be started. These highly redundant systems nearly eliminate the possibility of accessing the vehicle or starting the vehicle with the incorrect key. Unfortunately, they do not prevent breaking and entering, but will sound an alarm by horn and lights with unauthorized entry.

Attempting to start a vehicle with all of this technology to recognize the key should be seamless and simple. However, in the case of any portion of the anti theft system failing, even intermittently, the vehicle may become impossible to access or start. Some issues will cause the alarm to sound at random, disallow the key fob from accessing anything on the vehicle, sound the alarm anytime the hard key is turned in any lock cylinder, or allow the vehicle to be accessed but prevent operation. Also, some models will prevent the vehicle from being moved from part to any other range in an effort to prevent the vehicle from being driven or moved.