Faults in a car's onboard computer system have become one of the most common reasons drivers break down, according to the AA, with failures in the Body Control Module (BCM) more than doubling since 2020.
The BCM is the electronic hub that controls many of a vehicle's systems, including lights, windows, wipers, alarms and central locking. The AA says computer-related breakdowns now reach 36,000 a year, placing BCM problems fifth on its list of the top five reasons it was called out between July 1, 2025 and June 30, 2026.
The most common reason for an AA callout during that period was a failing 12-volt battery. Tyres were the second most common cause, followed by engine or clutch problems and then issues with the alternator or starter motor.
The trouble with BCM failures is that when the unit develops a fault, several systems can stop working at once, often with little or no warning to the driver.
What causes a BCM to fail
The reasons a BCM can start playing up are wide ranging, according to the AA. Driver error, a problem with the entry key chip, physical damage, botched garage work or a simple computer glitch can all trigger faults in the electronic control unit.
The BCM has nothing to do with a car's engine. Instead, it acts as the brain behind a vehicle's convenience and comfort features, managing everything from lights, wipers and windows to keyless entry, cruise control, heated seats, seat belts and the sunroof.
It works by receiving input signals from sensors around the car, such as a door switch or a light sensor, and then sending the correct signal out to actuators like the power window motor or the air conditioning compressor.
How a fault shows up
Because the system is so complex, a fault in the BCM can cause a car to malfunction in many different ways, from warning lights appearing on the dashboard to windows that will not go up or down.
BCM problems can also create security risks. Since the module powers a car's theft alarm, a fault could leave a vehicle more vulnerable to being stolen.
Getting it diagnosed and fixed
When a car is taken into a garage, a qualified technician will diagnose the problem using a tool connected to the vehicle's computer system, which reads out error codes stored by the BCM.
Drivers who are told their BCM is faulty are advised to get a second opinion, because it is an expensive component to replace. If the unit can be repaired rather than fully replaced, it will be both cheaper and quicker to fix.
