B117A airbag control unit fault

What does error code B117A mean?

Error code B117A indicates a specific hardware fault in the airbag control unit. It signals an electrical short circuit to ground in the control circuit of the solenoid for the passenger-side active headrest. This is an internal fault of the control electronics.

The active headrest is a safety system that moves forward during a rear-end collision to reduce the risk of whiplash injury. It is actuated by a solenoid controlled by the airbag control unit.

Code B117A is set when the internal circuit driving this solenoid experiences a permanent short circuit to the vehicle’s ground point. The control unit detects this abnormal condition and reports a critical fault that affects the entire SRS system.

How can you recognize error B117A?

The primary symptom of error B117A is a continuously illuminated airbag warning light (SRS lamp) on the instrument cluster. Often, communication between the diagnostic tool and the faulty airbag control unit also ceases, making fault analysis more difficult.

A lit SRS lamp is an unmistakable warning sign indicating a malfunction in the occupant protection system. As long as this error remains active, the airbags and other restraint systems may be inoperable.

Another typical indication is complete loss of communication. Workshops can no longer read the control unit to confirm or clear fault codes, as it no longer responds to requests via the diagnostic interface.

What causes B117A?

The main cause of error code B117A is an internal hardware defect on the airbag control unit’s circuit board. The short circuit to ground arises from the failure of electronic components in the specific circuit that drives the solenoid of the active headrest.

This is not a software issue or incorrect coding. The root cause lies in the physical electronics—such as a defective driver chip, a damaged transistor, or a faulty trace.

A thorough diagnosis is essential to confirm that the fault truly resides within the control unit. External factors, such as a defective cable or a short circuit in the headrest’s solenoid itself, must be ruled out before removing the control unit.

In which models does B117A occur?

Error code B117A specifically occurs in Mercedes-Benz vehicles, particularly in the airbag control unit of the ML-Class W164 series. At Airbag24, this fault is professionally and sustainably repaired for the affected models.

This repair is especially relevant for the following model:

Mercedes-Benz: ML-Class SUV (W164 series, model years 2005–2011)

Airbag24 possesses the necessary expertise and technical equipment to precisely analyze and restore control units of this vehicle generation.

How is error B117A professionally repaired?

Professional repair involves refurbishing the removed airbag control unit at a specialist facility. At Airbag24, the faulty component causing the short circuit is located on the circuit board, expertly replaced, and the unit then thoroughly tested for full functionality.

Upon receipt of the control unit at Airbag24, a comprehensive incoming inspection is performed to verify error B117A and rule out any additional potential defects.

The actual repair focuses on eliminating the short circuit at the component level. Using specialized measuring instruments, the defective component in the active headrest control circuit is identified and replaced. If necessary, crash data stored in the control unit is also erased.

Following successful restoration, the control unit undergoes a final functional test on the test bench. This ensures the fault has been resolved and the unit operates flawlessly once again. Two years' statutory liability for defects is provided for the completed repair.