What does the body control module do?

It is the hub for most of the electrics you interact with every day rather than the ones that make the car go. Depending on the vehicle it operates the exterior and interior lighting, indicators and hazards, wipers and washers, central locking and the key fob, the horn, electric windows, the heated rear window and often the alarm and immobiliser. It coordinates all of that over the network with the other modules, which is why losing communication with it produces such a scattered and confusing set of symptoms.

Can water damage a body control module?

Yes, and it is the leading cause of these failures. The module is frequently mounted low in a footwell or behind a kick panel, which is exactly where water collects when a scuttle drain blocks, a door vapour barrier tears, a windscreen or sunroof seal fails, or a heater matrix begins to weep. Corrosion works its way into the connector and the circuit board. Fixing the leak matters as much as fixing the module, because a replacement dropped into a wet footwell will go the same way.

How many MOT failures can a body control module fault cause?

Potentially a long list, and this is the code where the total gets alarming. The tester checks headlights, side lights, indicators, hazard warning lights, stop lamps, the horn, and the wipers and washers directly, and every one that does not work is a defect on its own. Several of those are major. Add whatever dashboard warning lamps have appeared and a single water-damaged module can produce a failure certificate with half a dozen items on it, all from one cause.

Why won't my car start with U0140?

Because on most cars the body control module is part of the immobiliser chain. When you present the key, the module reads the transponder, checks it is a valid key for the car and passes that confirmation to the engine control unit, which will not allow fuelling or ignition without it. If the body module is not communicating, that confirmation never arrives and the engine control unit assumes the car is being stolen. The engine cranks perfectly well and simply never fires.

Network code · standard

U0140No communication with the body control module

Standard definition: Lost Communication with Body Control Module

Last updated 27 July 2026

Quick answer

The car has lost contact with the module that runs most of its everyday electrics. Lights, wipers, indicators, central locking and the horn can all stop working, and several of those are MOT items in their own right.

How the car works this out

The body control module is the hub for the car's comfort and convenience electrics, and it also handles the immobiliser on many vehicles. It exchanges messages constantly with the instrument cluster, the engine control unit and the door modules. U0140 is stored when those messages stop. What makes this code different from the others in this group is the breadth of what fails with it: this is not one system going quiet but a great many small functions disappearing at once, several of which a tester will check directly.

What you might notice

Exterior lights, indicators or hazard lights not working
Wipers or washers dead, or running when they should not
Central locking and the key fob not responding
Horn not working
Interior lights staying on or refusing to come on
Car refusing to start because the immobiliser cannot be satisfied
Warning lamps across the dashboard

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1Water ingress into the module, which is often mounted in a footwell or behind the kick panel where leaks collectCommon
How to check: Press the carpet in both front footwells firmly. Damp carpet above or beside the module is the single most common cause, and the leak must be found before anything is replaced.
2A blown fuse or a corroded fuse box terminal supplying the moduleCommon
3A corroded or partly unlatched module connectorCommon
4Aftermarket wiring spliced into the module's circuits — alarms, trackers, stereos, towbar electrics or parking sensorsPossible
How to check: Ask what has been fitted to the car and by whom. Poorly installed accessories are a leading cause of body electrical faults.
5Low system voltage from a failing battery or alternatorPossible
6A failed module, which usually needs coding and immobiliser programming when replacedPossible

What to do about it

1Check your lights, indicators, wipers, washers and horn before driving. Several of these are legal requirements as well as MOT items, and driving without working lights or indicators is an offence quite apart from being dangerous.
2Check the footwells for damp. Water reaching a body control module is the leading cause, and a replacement fitted into a wet footwell will fail the same way.
3Find the leak — a blocked scuttle drain, a failed door vapour barrier, a leaking windscreen or sunroof seal, or a heater matrix — before any electrical repair.
4Have the battery load tested, because low voltage produces exactly these symptoms.
5Declare any aftermarket electrics fitted to the car, which will save a great deal of diagnostic time.

Can you keep driving?

This is one of the few chassis codes where the answer is a clear no, and it is not because of anything under the bonnet. If your indicators, brake lights, headlights, wipers or horn are not working, the car is not legal to use on the road and is genuinely dangerous to other people. Check each of those functions before you move the car, and if any are missing, have it recovered rather than driven.

Will it fail an MOT?

This code can produce a longer list of defects than almost any other single fault, because the body control module operates so many of the things a tester checks by hand. Headlights, side lights, indicators, hazard warning lights, stop lamps, the horn and the windscreen wipers and washers are all inspected directly, and each one that does not work is a defect in its own right — several of them major. Add whatever warning lamps have appeared on the dashboard, and a single water-damaged module can produce a failure certificate several items long.

Common questions

What does the body control module do?: It is the hub for most of the electrics you interact with every day rather than the ones that make the car go. Depending on the vehicle it operates the exterior and interior lighting, indicators and hazards, wipers and washers, central locking and the key fob, the horn, electric windows, the heated rear window and often the alarm and immobiliser. It coordinates all of that over the network with the other modules, which is why losing communication with it produces such a scattered and confusing set of symptoms.
Can water damage a body control module?: Yes, and it is the leading cause of these failures. The module is frequently mounted low in a footwell or behind a kick panel, which is exactly where water collects when a scuttle drain blocks, a door vapour barrier tears, a windscreen or sunroof seal fails, or a heater matrix begins to weep. Corrosion works its way into the connector and the circuit board. Fixing the leak matters as much as fixing the module, because a replacement dropped into a wet footwell will go the same way.
How many MOT failures can a body control module fault cause?: Potentially a long list, and this is the code where the total gets alarming. The tester checks headlights, side lights, indicators, hazard warning lights, stop lamps, the horn, and the wipers and washers directly, and every one that does not work is a defect on its own. Several of those are major. Add whatever dashboard warning lamps have appeared and a single water-damaged module can produce a failure certificate with half a dozen items on it, all from one cause.
Why won't my car start with U0140?: Because on most cars the body control module is part of the immobiliser chain. When you present the key, the module reads the transponder, checks it is a valid key for the car and passes that confirmation to the engine control unit, which will not allow fuelling or ignition without it. If the body module is not communicating, that confirmation never arrives and the engine control unit assumes the car is being stolen. The engine cranks perfectly well and simply never fires.

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