BMW BST Symptoms. What Is It And How To Repair?

BMW cars are often thought to be not very reliable. They require costly maintenance and they often ask you to go to a service station or to the dealer. But actually, these cars are designed very wisely and they save the lives of people. A lot of special systems in your BMW are designed to reduce the chances that you will get injuries or even die in case of an accident.

Yes, sometimes these systems malfunction and they need to be serviced a little in order to function properly again. But this investment is good for your safety and life. If you don't want to spend a lot of money on your car, don't buy BMW. These German vehicles love when you don't try to save up some funds during a regular service checkup or repair.

Today, we are going to talk about the BST system (Battery Safety Terminal):

1. What is BST and why do you badly need it in your BMW?
2. What are the symptoms of a malfunctioning BST sensor?
3. How to deal with the situation?
4. How much is to repair your car when a BST sensor is broken?

Let's get started!

What is BST and how is it meant to work?

Your BMW is likely to have a lot of functions and units that are called with strange words and abbreviations. BST is just one of them. It stands for "Battery Safety Terminal".

This system works to cut off the battery of your car once the frame and body of the vehicle are damaged in an accident. The main task of this action is to avoid a short circuit when an accident happens. You may have seen cars burning after an accident. In most cases, this happens because of a short circuit when the battery keeps on supplying the system with energy.

So, the BST system works like this:

• there is a sensor near the battery that gets signals from different safety sensors in your vehicle;
• once your BMW gets into an accident, some sensors feel this and send signals to a BST sensor;
• the BST sensor cuts off the battery so it's safe now and it can't start burning or explode;
• the battery remains turned off from the electric system of a car until the BST is repaired.

As you see, the system is pretty simple. It works to cut off the battery, so you will not be able to use any units in your vehicle. This is purely a safety feature that will help you save your life, health, and your possession in case of an accident. The trouble with the BST sensor is that it can start malfunctioning and turning off the battery without any accidents. Especially on older cars that have big mileage where electronics starts working with failures.

What are the main symptoms of a malfunctioning BST sensor?

To deal with the problem you will need to understand where it starts in your car. Well, the best option is to go to a service station. But being able to locate the problem in your car may save you a lot of money.

Here are the symptoms of a malfunctioning BST system in your BMW:

1. Your car won't start. When you turn the key into the needed position, nothing happens. Or when you push the start button, the starter doesn't even try to rotate the engine.
2. The ignition doesn't turn on. When you turn your key, lights don't go on, no sensors start working on the dashboard. Your car seems to be completely dead.
3. The alarm system and central lock don't work. Those systems depend on your battery, so they will not work. Some vehicles have a reserved battery for the safety system and locks, so they may still work fine. But in most cases, your car is dead.
4. When you try to complete diagnostics, you can't connect the devices to your car - it doesn't show anything.
5. All these symptoms will show the BST problem, especially after a collision (even a slight collision) or after you saw the airbag light on the dashboard.

For the BST sensor to cut off the energy supply you will not necessarily need to hit your car hard. Even a small accident like hitting a wheel over the curb may result in a BST sensor cutting the battery off. And you will want to know what to do in this situation.

Where is the BST sensor located?

We can tell you that the BST system is located on the positive battery clamp in your BMW. But actually, it doesn't make things better for you. If the sensor triggered the battery cut-off, you will not be able to repair it on the go. But we still recommend you look at the clamp. It's not always about the sensor. Maybe the clamp is loose and you can just tighten it to get your car going.

Unfortunately, in most cases, it's all about the sensor. Especially if you got into an accident, even the minor one. Checking the system on the go is impossible. Even if you have the equipment for diagnostics with you all the time, the vehicle will not show a thing because of the energy cut-off. So the only thing you may do in this case is to call a tow truck and take your vehicle to a service station.

Why can a BST sensor trigger the energy cut-off without an accident?

Firstly, you will need to remember whether you had any accidents within 2 or 3 days. In a vast majority of cases, your BMW should have had an accident before the BST sensor came into action. Maybe, someone took your car and had a minor accident. Just check it thoroughly because this makes 95% of cases when a BST sensor triggers the battery cut-off operation.

If you don't remember any accidents, you should think about other cases that could have triggered the sensor and be the reason for your problems. Here are some ideas:

• the accident sensors could malfunction - your BMW has a lot of them, and sometimes they send wrong signals to your vehicle's safety system including your BST sensor;
• the airbags electronics may have gone wrong - this may easily lead to problems with the BST as all the safety features in your BMW vehicle are connected into one module;
• the BST sensor itself may be faulty - this may happen in vehicles that are quite old and have high mileage on the clock, but things happen;
• someone may have hit the sensor when servicing your car - this could easily lead to a sensor malfunctioning and cutting off the battery in the car.

You may not be sure that the problem is in the sensor. The same symptoms may come when a battery is flat or when wiring has problems. Sometimes, a battery terminal gets dirty and energy flow stops. The same symptoms will be always when a battery gets cut off from the electric system of your car. But the BST sensor is still the first thing we would check in this case. Especially, if you can remember an accident or even a hard hit over the curb.

How to repair the BST in your car?

Unfortunately, you will not be able to do anything with this sensor on the go. The only way is to go to a service station or to the BMW dealer. They know this problem and will do everything to help you solve it fast. BST sensors are not very hard to replace. You can even do it by yourself if you know something about cars and how they work. The sensor is located on the positive clamp which is not hard to locate.

Here's what a professional mechanic will do:

1. Locate the positive terminal and the BST sensor itself. Have a look at the sensor and try to understand if it is damaged or not.
2. Find an appropriate BST system sensor in a BMW part shop or online. Be careful - BMW made a lot of modifications to the system, so you will need to use the VIN code to buy the parts.
3. Take off the old sensor from the clamp, look at it and try to see any damages, bad electric wiring, etc.
4. Put a new sensor and fasten it according to the instruction. The battery clamp should be taken off the post.
5. Get the clamp to the post and make sure it's fastened there. Try to start your car and see if there is a check engine error or any other messages on your dashboard.

This problem is not very hard to deal with. But choosing a proper sensor and sometimes even locating it can still be problematic.

Final words

Fortunately, the BST sensor is not the most expensive part of your BMW vehicle. This is the safety system that cuts off the battery in case of an accident. This saves your car from fire and it can save your life in a bad accident. If you think that the system is malfunctioning, you should replace the sensor with a new one. Always buy original BMW parts in such cases because aftermarket sensors can bring new problems.