Immobilizer

A transponder chip in the key, or the keyless entry fob, sends a code when the key is near the ignition. The immobilizer control unit checks it and only then allows the engine control unit to run the starter, fuel and ignition. If the code is not recognised, the engine either does not crank or starts and stops after a second or two. A security light on the dashboard usually flashes or stays on.

Immobilizers became widespread from the 1990s, and in the European Union they have been required on new passenger cars since the late 1990s.

An antenna ring around the ignition lock or in the cabin powers the transponder and reads its code. The control units exchange encrypted messages before releasing the engine. An antenna ring around the ignition lock or near the start button reads the key’s chip. Only when the code is accepted does the control unit allow fuel injection and ignition.

Weak key batteries, damaged keys, new keys that have not been programmed and wiring faults cause no-start problems. Try the spare key first: if it starts, the original key is the problem. A flashing key or security symbol often means the key is not recognised. Try the spare key; new or replacement keys must be programmed to the car.