What is BCM in a Car?

It is common for car owners not to know what their car's BCM is. That can make it quite confusing when your mechanic tells you that your BCM is bad or that the problem is related to your BCM and may need to be replaced. 

Your BCM, or Body Control Module, is an electric unit installed in your car designed to handle many of your vehicle’s electric features, including the power windows and central locking.

As you might imagine, the BCM is a relatively important part of your car. It might not always be essential, but your BCM controls a lot of your vehicle’s luxury features that help keep you comfortable. Your BCM may be critical for providing additional alerts and warnings in some cars, like your check engine light or oil lights. Many of the alerts and notifications on your dashboard are directly tied to the BCM. 

Your BCM often communicates with other important control modules in your vehicle. That means that, depending on how your BCM is malfunctioning, your car may start to have other problems as well. Strange codes or reduced performance that can’t otherwise be explained may be indicative of a malfunctioning BCM. 

Even though it’s one of your vehicle’s lesser-known parts, the BCM is one of the most important electronic components in your vehicle. 

What Does Your BCM Do?

The exact responsibilities of the BCM vary quite a bit from car to car. Luxury vehicles generally have more things for the BCM to handle, while cars with fewer features can have a less powerful BCM to manage the smaller number of tasks. 

Here’s a basic list of the things your BCM handles. Some vehicles will have more responsibilities than this, and others will have fewer. 

Warning Lights

This is probably the most critical function of your BCM, and malfunctioning warning lights can sometimes be an early sign that your BCM is starting to fail. Your BCM doesn’t typically monitor the different parts of your car. Still, it’s a hub that communicates with those other parts and receives information to determine when a warning light is necessary. 

The check engine light is one of the most important of these indicators, but other warning indicators like the oil warning are often equally important. If your vehicle provides low tire pressure warnings or tells you when the windshield wiper fluid is running low, those warnings will also run through the BCM. 

Less important warnings can often show problems with a BCM along with the more important ones. For instance, your wiper fluid indicator might turn off for no reason without you filling the reservoir, especially if it has turned on and off several times for no reason. Unfortunately, like the check engine light, other more important indicators can have problematic errors that might mean the light is turning on and off for legitimate reasons. 

Air Conditioning

Your BCM is also in charge of controlling your air conditioning and is designed to monitor input from your dash, where you set the temperature and the rate of your fans blowing. It then communicates with the air compressor to make sure there is enough cold air available. 

Your BCM may also be involved in running your fans when the air conditioning isn't on and stopping the air compressor when you switch over to heat. 

Powered Locking System

Your BCM also controls your car’s locking system if it uses an electric lock. If you use a key fob to start your vehicle remotely, the BCM controls that as well. Internal lock buttons are also controlled by the BCM, which is why you can selectively lock and unlock specific doors in most modern vehicles. 

Keyless Entry and Remote Trunk Opening

Your BCM also handles distance requests from your fob, like keyless entry where it will unlock your doors as you approach or letting you open your trunk with a button on your car’s fob. The BCM works with keyless detection units and coordinates signals between different systems to ensure your car reacts appropriately. 

Basic Electrical Systems

Your BCM also controls some relatively simple quality of life functions that are important for keeping your comfortable and safe while you’re driving. Things like controlling your windshield wipers and adjusting your side mirrors are up to your BCM. 

If you’re driving a vehicle with electric seat adjustments, the BCM will control those adjustments as well. 

Symptoms of a Failing BCM

With such a critical part, it’s vital to notice signs of possible failure before your failing BCM can start to impact more critical functions, or even just fail to send an appropriate warning signal when something else goes wrong. 

One of the first signs of a failing BCM in luxury vehicles is when the keyless entry system stops working. If your car relies on a fob for keyless entry, it’s essential to make sure the fob’s batteries haven’t run down, but if fob problems have been eliminated, you’ll likely need to look at your BCM as the next culprit. 

Other problems, like your windshield wipers not coming on or coming on randomly, can both be signs that your BCM is starting to have problems. 

You should also look for other signs that the BCM is starting to have problems, like flickering or random dash alert lights. If your air conditioner starts to have issues and the air conditioning system itself isn’t damaged, it may also be a sign that it’s receiving bad signals from your BCM. 

One of the more serious signs of BCM failure is bad or uneven acceleration. If your BCM is sending bad signals or isn't sending correctly timed signals to the other systems in your car, then acceleration is one of the first places you'll see significant performance changes. 

There is some good news in all this, though. The BCM is a common enough repair that the part is relatively affordable and easy to get. The BCM is also a reasonably easy part to replace, so many car owners can replace the BCM on their own at home. Mechanics also replace BCMs fairly regularly, so you can usually get a repair without having to spend too much on it. Most BCMs cost a couple of hundred dollars for the part, and labor costs are typically similar.