B240 airbag control unit fault

What does error code B240 mean?

Error code B240 indicates a malfunction or defect in the airbag control unit, a critical safety component. The vehicle’s electronic system detects this fault and stores it in the fault memory. It points to issues within the control unit itself or in connected components such as sensors or wiring.

This error code belongs to the family of airbag-specific diagnostic trouble codes (DTCs) under the body electronics category (Body - B). The airbag control unit continuously monitors all components of the restraint system. When an inconsistency or defect is detected, the system logs code B240 to signal the need for maintenance.

In addition to B240, more specific error codes such as 93F9 (control unit malfunction) or 93D7 (airbag control unit functional failure) may also be stored. These often provide more precise indications of the nature of the internal defect.

How can you recognize error code B240?

The primary and unmistakable symptom of error code B240 is the permanently illuminated airbag warning light on the instrument cluster. This indicates that the entire airbag system has been preventively deactivated and will not deploy in the event of a crash. Consequently, passing the main vehicle inspection (HU) becomes impossible.

The red airbag warning light remains steadily lit after starting the vehicle and does not extinguish as it normally would. This is a clear sign that the control unit has detected a safety-critical fault and switched the system into emergency mode.

In some cases, additional issues with vehicle diagnostics may occur. Attempts to read the airbag control unit using a diagnostic tool may fail, as the internal fault can also interfere with communication via the OBD interface.

What are the causes of error code B240?

The causes of B240 are varied, ranging from electronic defects in the control unit due to short circuits or moisture to communication disruptions caused by faulty connectors. Damaged components such as seat occupancy mats or crash data that has not been erased after an accident are also common triggers.

A frequent source of errors is electronic damage directly on the control unit’s circuit board. Such damage can result from voltage spikes, a defective vehicle battery or alternator, or even moisture ingress. Control units installed beneath the seats are particularly vulnerable to damage from spilled liquids.

Communication problems caused by poor connector contacts or broken cables can also trigger the fault. Defective airbag connectors—especially in three-door vehicles where the front seats undergo constant movement—are a well-known weak point.

Peripheral components of the airbag system, such as a faulty seat occupancy mat, malfunctioning sensors, or issues with seatbelt buckles, can likewise prompt the control unit to report an internal error and store code B240.

Following a crash, so-called crash data is permanently saved in the control unit’s memory. If this data is not professionally and completely erased, the control unit remains locked, and error code B240 cannot be cleared.

How is error code B240 diagnosed?

The diagnosis of error code B240 is multi-step. It begins with a visual inspection of the wiring and the control unit. Next, the fault memory is read using specialized diagnostic equipment to identify code B240 along with any accompanying errors. Finally, the exact cause is analyzed.

First, a thorough visual inspection of relevant components is performed. The technician checks the wiring for visible damage, examines connector contacts for corrosion or loose connections, and inspects the control unit itself for signs of moisture ingress or physical damage.

In the second step, the vehicle’s fault memory is read using an appropriate diagnostic system via the OBD‑II interface or vehicle‑specific ports. This confirms error code B240 and lists any other stored faults that aid in determining the root cause.

Based on the retrieved data, targeted root-cause analysis begins. Electrical circuits are tested, and sensor functionality is verified to pinpoint the exact source of the problem before removing the control unit for repair.

Which vehicles are affected by B240?

Error code B240 frequently occurs in specific vehicle models, especially BMWs. At Airbag24, this error code is specifically repaired for the BMW M6 E63 Coupe model from 2005 to 2010, where it is reported more often.

At Airbag24, the B240 error code is repaired for the following specific model:

BMW M6 E63 Coupe (2005–2010)

In the BMW M6 of the E63 series, the control unit is typically located in the passenger footwell. It should be noted that various versions and manufacturer part numbers of this control unit exist for this vehicle model, which must match exactly in case of replacement.

How is the B240 error repaired?

Professional repair of the B240 error is typically performed as a mail-in repair of the removed control unit. Depending on the cause, stored crash data is erased, the fault memory is reset, or firmware adjustments are made. Each repair at Airbag24 comes with a 12-month warranty.

After removal, the defective control unit is sent to Airbag24. There, a detailed component-level diagnosis is carried out to identify the exact root cause of the fault. This process ensures that not only the error code but also the underlying issue is resolved.

A common repair step involves erasing crash data using specialized software. If hardware components are defective, they are professionally replaced. In some cases—such as with BMW vehicles—coding adjustments may also be necessary using tools like NCS Expert to restore full functionality.

If electronic repair is no longer feasible, a replacement unit can be used. In this case, it is ensured that the spare part meets the correct specifications and is subsequently re-coded for the specific vehicle.

Following successful repair and a final functional test, the control unit is returned. After reinstalling it in the vehicle, the fault memory must be cleared again. A 12-month warranty covers all work performed.

Important Safety Notes Regarding the Airbag System

Work on the airbag system is safety-critical and must be carried out exclusively by trained professionals. The use of simulators or jumpers is illegal, life-threatening, and results in the revocation of the vehicle’s operating permit. Repair is essential for safety.

Under no circumstances is it permissible to use so‑called airbag simulators, resistors, or jumper plugs. Such manipulations falsely indicate non‑existent functionality to the system, are life‑threatening, and immediately invalidate the vehicle’s general operating permit (ABE).

Due to the pyrotechnic components within the airbag system, all work—including removal and installation of the control unit—must be performed only by individuals with appropriate expertise and in a qualified specialist workshop.