The body control module (BCM) is a type of computer that controls many of the electrical systems around the interior and exterior of the vehicle outside of normal engine operation. It allows you to operate windows, headlights, taillights, fog lights, door locks, HVAC controls, and much more.
When you have a malfunctioning BCM, many of these systems will work intermittently or fail to operate altogether. Here are some of the most common symptoms of body control module failure and how much replacing it will cost you.
Symptoms of a Bad Body Control Module
1) Dead Battery
A bad electrical connection inside the body control module may cause a parasitic draw. A parasitic draw occurs when a vehicle system draws power from the battery, even when that system is supposed to be powered off.
If your battery dies after a day or two of the vehicle sitting, you likely have a parasitic draw. There are ways you can find the cause of a parasitic draw using a multimeter. Parasitic draws are hard on the battery and the alternator due to the constant charging that needs to happen each time you drive the vehicle.
Some vehicle batteries die after weeks of sitting. However, this can be considered normal behavior. Many vehicle systems are designed to draw a small amount of power. The very small amount of power draw allows some system settings to persist even when the vehicle is off, such as your clock and radio settings.
2) Buttons Stop Working
The body control module receives the signals from buttons and switches you operate inside the cabin. If you notice some or all of the buttons in the interior quit working, this is a sign of a bad BCM.
Note: If only buttons on the steering wheel quit working, you may have a bad clock spring instead.
3) Blown Fuse
An electrical short may cause an unsafe level of current for a given circuit. Fuses are designed to prevent this excess current from damaging the circuit or causing a fire.
If you replace a fuse and the replacement also blows, you have an electrical fault that needs to be addressed. If the circuit with the blown fuse is connected to the body control module, try swapping out the body control module to see if the issue persists.
Make sure you replace the fuse with one that is designed for the same level of current. For instance, a 15 amp fuse should only be replaced with another 15 amp fuse.
4) Electrical Gremlins
“Electrical gremlins” is an informal term used to describe erratic electrical issues that are often intermittent or inconsistent. Electrical gremlins are typically caused by shorts, bad grounds, and poor connections.
When you drive the car over bumps, the sudden movement may knock electrical connections loose and briefly disrupt the normal flow of current. Inconsistent voltages, or weak signals that are in between expected values can cause intermittent electrical problems to present themselves in an otherwise functional system.
5) Limp Mode
Body control modules often have a feature called a limp home function. Whenever a BCM detects a fault in a critical system or process, a limp home signal can be sent to the ECU.
The limp mode signal informs the ECU of a problem with the vehicle. The ECU may then limit certain driving characteristics of the vehicle, allowing the driver to make it safely to a shop for repairs.
6) Brake Lights Stay On
If there is a fault in your brake light operation, the brake lights often stay on solid, even when you’re not pressing on the brake pedal.
Brake lights are operated by the body control module. If you’ve already checked your brake switch and are sure it is working, check the body control module and the brake light pins that connect to it.
7) Headlights Don’t Work
The headlight circuit is typically controlled by the body control module. If both headlights quit working at the same time, check the body control module. This is especially true if other lights around the vehicle quit working at the same time.
Body Control Module Replacement Cost
If you do the work yourself, you can expect to pay around $150 to $200 for a new BCM. The labor cost will be around $300 (approximately 1-2 hours), depending on the location of the body control module in your vehicle.
Total cost at an independent mechanic will be around $500.
Where Can I Find a Replacement Body Control Module?
Refurbished body control modules can be sourced from salvage yards or eBay for less than $200. eBay vendors often have excellent return policies if you discover a replacement BCM did not resolve your issue.
If you find a known working replacement module that does not resolve your issue, the issue may be as simple as clearing or repairing some bad wiring connections.
This may involve de-pinning the electrical connectors that plug into your BCM. De-pinning kits are often vehicle specific but affordable generic kits exist for as little as $20.
R50 and R53 Mini Cooper Body Control Module Failures (Water Ingress)
Some vehicles are especially prone to issues with the body control module. One example of this is the R50 and R53 Mini Cooper, which range from model years 2002 to 2006.
The body control module on the R50 and R53 Mini Cooper is located in the passenger side footwell (which is the driver side in the UK). It is placed underneath the A pillar. The weather seals around the A pillar are prone to leaking in these vehicles. If water leaks into the cabin from the A pillar, it will leak down into the passenger side footwell.
If the issue is left unchecked, the body control module will be subjected to water ingress. This water ingress corroded connections and may cause electrical shorts. The solution is to replace the body control module if it is faulty. Sometimes electrical wiring will need to be cleaned or re-pinned, depending on the condition of the wiring.
This is a known and documented issue you can find on Mini forums and YouTube. In some cases, the issue was able to be fixed under warranty. If the vehicle is out of warranty, you may need to source a replacement body control module to resolve the issue.
Before you replace the BCM, make sure you fix the water ingress issue so you don’t flood your new module. There are many places water tends to leak from, so check all of your seals. A few common leak locations are the weather seals around the doors and the seals inside the engine bay (around the ABS module and brake fluid reservoir).
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