A problem BMW owners may face is that the car will not start after an accident. This problem is often caused by the BMW Battery Safety Terminal (BST sensor) fault mounted on the positive terminal.
BST sensor is often confused with the intelligent battery sensor (IBS), which is mounted on the negative terminal.
Symptoms of bad BST
Symptoms of bad BST sensor include:
- BMW won't start
- BMW will not start after an accident
- If airbags are deployed, BMW won't start.
- Accessories and lights STILL work.
- The airbag light is on.
If you are stranded on the road and can't start your BMW, check to make sure the battery's positive cable is not loose. If it loses, the BST ignited and disconnected the positive.
How to Replace the BMW Battery Safety Sensor
What you will need
- Park the BMW on level ground, turn off the ignition and engage the parking brakes.
- Press the trunk release button and open the trunk.
- Remove the plastic retainers on the right side of the trunk. This will allow you to remove the right panel and access the battery safety terminal. On a station wagon, the battery will be mounted on the floor of the trunk.
- Use a 10mm open-end wrench to disconnect the negative battery terminal.
- Disconnect positive battery terminal.
- Remove the old BST sensor. First, you will need to unplug the two black wires which are for the airbag. Next, remove the 12mm nut that connects the accessory cable to the battery clamp.
- Install the BMW BST sensor. Connect the two wires for the airbag module.
- Connect positive battery terminal and lastly connect the negative terminal.
- Locate the diagnostic port under the dashboard.
- Connect any scanner that can read airbag fault codes, such as Foxwell NT510 for BMW.
- Clear BMW fault codes stored in the airbag module.
Keep in mind that a basic code reader can not clear BMW airbag codes.
Bypassing BMW BST Sensor
CAUTION! This fix is not recommended.
If the BST sensor exploded (ignited), it SHOULD be replaced. Some BMW owners decide to bypass the BST sensor by cutting the wires to the sensor and installing a resistor. This allows you to clear the airbag light but disables a key safety feature.
Two small wires that go to the BST sensor (near battery positive terminal) are shorted with a resistor to bypass the BST sensor. A 2-ohm resistor is installed at the end of the wires bypassing the BST sensor. This will emulate a working BMW sensor, and the airbag light codes are reset using a BMW scanner.
When you install a BMW BST bypass sensor, you remove a safety feature and increase the chances of a fire if you get involved in an accident. You can replace the sensor yourself.
In the past, it would cost anywhere from $500 to $900 to replace the BMW battery safety terminal. This is why some BMW owners would bypass the BST sensor and connect the positive cable directly to the battery positive terminal.
By deleting the BMW BST sensor, you will be disabling a safety feature of your BMW. Today you can get a BMW BST repair kit which you can order on eBay for a fraction of the original prices.
Another solution is to remove the BST from a used BMW and install it on yours. You need to make sure that the BST is not from a BMW involved in an accident.
Where is the BMW battery sensor?
BST sensor mounts at the end of the battery positive cable and is part of the battery positive clamp that attaches to the battery post.
What does BMW Battery Sensor do?
It is designed to disconnect the battery when the car is involved in an accident.
Which BMW models have an intelligent battery sensor?
BMW introduced the Battery Safety Terminal in the early 2000s.
Can I delete the battery sensor on a BMW?
Yes, but we don't recommend deleting the BMW BST sensor. If you decide to bypass the BST sensor and are involved in an accident, the battery will stay connected, increasing the chance of a fire or explosion caused by a battery short.