Q: How long can an ECU last? I may extend the certificate of entitlement on my BMW and would like to know about the longevity of this crucial component. The ECU or electronic control unit is essentially a computer which controls various aspects of a car's functions.

A: For example, it switches on the engine cooling fan when a certain temperature is detected.

These days, most cars have more than one ECU, each dedicated to controlling a specific system such as the transmission, power windows and door locks or air-conditioning.

But what you are probably most concerned about are the engine and transmission ECUs. While physically separate, they communicate with one another continuously to effect the optimal responses at any given moment.

No manufacturer will suggest an ECU replacement after a certain mileage or age.

Generally, ECUs are quite robust. A malfunction is usually the result of a faulty sensor. When this happens, the ECU processes a false signal, thus providing a wrong output.

It takes time, effort and quite a bit of expertise to isolate such a fault.

An ECU failure is rare and can be caused by a serious collision, dampness from being submerged in a flood, or an electrical short circuit caused by poor wiring when installing accessories.

If your car has not had ECU-related faults before, it is unlikely one will arise in the next 10 years. This does not mean it will not happen - just that it is less likely.