Immobilisers and Car Alarms: Why They Fault and What Can Be Done

What the immobiliser actually does

An immobiliser is an anti-theft system that stops the engine running unless it recognises the correct key. A small transponder chip in the key, or a coded signal from a keyless fob, is read by an antenna around the ignition. The security module checks the code and, if it matches, lets the engine management system allow the car to start.

If the code is not recognised, or the signal cannot be read, the engine may crank but not start, or may start and cut out after a second or two. A security light on the dash usually flashes or stays lit to show the system has not accepted the key.

Common reasons a car will not start with the security light on

Many causes are simple. A flat or weak car battery can stop the security system and starter working properly, and low voltage can confuse several modules at once. A worn or damaged key, or a transponder chip that has failed after being dropped, can mean the car no longer recognises it. A weak battery in a keyless fob can make it unreadable.

Other causes include a fault in the ignition antenna, damaged wiring, a problem in the security module itself, or a mismatch after other work has been done on the car. Trying the spare key is a quick test that helps tell the difference between a key problem and a car problem.

• Try the spare key first
• Check the car battery and the fob battery
• Remove other keys or tags from the key ring, as they can interfere
• Note exactly what the dash light does and tell us

Alarms going off for no reason

A car alarm is triggered by sensors: door switches, boot and bonnet latches, and in some cars interior movement or tilt sensors. If any of them wrongly reports that something has been opened, the alarm can sound.

Bonnet and door latch switches are exposed to water, dirt and road salt, and they corrode or stick. A weak or ageing battery is another classic cause. When voltage dips, the alarm module can read it as tampering. Faulty siren units, damaged wiring and a sensitive interior sensor, perhaps set off by something swinging from the mirror, can all produce false alarms.

• Door, boot or bonnet latch switch stuck or corroded
• Low or failing battery
• Wiring damaged where it passes through door or boot hinges
• Interior movement or tilt sensor set too sensitively

How the problem is diagnosed

The first step is to read the car's stored fault codes with diagnostic equipment, as the security and engine systems often record why they refused to start the engine. We then check battery condition and charging, because voltage problems are so common, and test the key, antenna and wiring.

For alarm faults we look at each sensor circuit in turn. Live data can show whether a door or bonnet switch is reporting open when it is closed. At Banks of the Tyne in Hebburn we carry out diagnostics, electrical fault finding and wiring repairs, and we work on alarms and immobilisers as part of that.

What can be done

Depending on the findings, the fix might be a new battery, a replacement latch switch, a wiring repair or a replacement key or antenna. Some jobs need keys to be coded or programmed to the car. Whether a particular unit can be repaired, replaced or reprogrammed depends on the vehicle and an inspection, so we do not promise an outcome before we have looked.

If you are considering fitting an aftermarket alarm or immobiliser, speak to us first about what is suitable for your car. Poorly fitted extras can cause the very battery drain and false triggers described above. Insurance requirements are a matter for your insurer, so check with them before changing anything.

Frequently asked questions