B0138 airbag control unit fault

What does error code B0138 mean in the airbag control unit?

Error code B0138 indicates a specific fault in the electrical circuit of the pyrotechnic seatbelt pretensioner. The airbag control unit (SRS) detects an electrical issue—typically a short circuit, a broken wire, or a defective sensor—which results in deactivation of the affected restraint system and compromises occupant safety.

Technically, the control unit continuously monitors the resistance value of the igniter in the seatbelt pretensioner. If this value deviates from the predefined reference values, error code B0138 is stored in the fault memory. This is a critical safety measure designed to prevent unintended deployment or failure during a crash.

The system then switches to a safe mode, indicated by the illumination of the airbag warning light on the instrument panel. As a result, the functionality of the entire airbag system may be impaired.

How can you recognize error code B0138?

The primary and unmistakable symptom of error code B0138 is the persistent or intermittent illumination of the airbag warning light (SRS lamp). This clearly signals to the driver that there is a problem with the restraint system and that immediate diagnosis is required, as the protective function is no longer available.

After starting the vehicle, the airbag control unit performs a self-test. If error code B0138 persists, the warning light remains illuminated and does not extinguish after the usual few seconds.

Other driving-related symptoms typically do not occur. The only indication of the problem is the warning light and the error code stored in the fault memory, which can be read using an appropriate diagnostic tool.

What causes error code B0138?

The causes of B0138 are almost exclusively hardware-related and stem from the electrical activation of the seatbelt pretensioner. First and foremost are defective cables or connectors due to corrosion, moisture, or mechanical damage. A faulty seatbelt pretensioner sensor itself, or an internal short circuit within the airbag control unit, can also trigger this error.

Defective cables and connectors are a common source of failure. Especially under the seats or at body penetrations, cables can break due to constant movement or aging. Corrosion caused by ingress of moisture increases resistance and likewise leads to fault detection.

A defect in the seatbelt pretensioner sensor itself is another possible cause. Internal windings or electronics may fail, transmitting incorrect readings to the control unit.

Rarer but more serious is an internal fault within the airbag control unit. A short circuit on the circuit board—often caused by water or moisture ingress—can damage the output to the seatbelt pretensioner. Software errors or communication issues between components may also play a role in some cases.

Which vehicles are affected by B0138?

Error code B0138 occurs frequently in specific Lexus models, particularly within the RX series. This suggests shared design features among the installed components or cable routing in these vehicles. At Airbag24, this error code is specifically repaired for the affected models.

At Airbag24, error code B0138 is diagnosed and repaired for the following Lexus models:

Lexus RX RX300

Lexus RX AL20 series (model years 2015–2019)

Lexus RX AL20 facelift series (model years 2019–2022)

How is error code B0138 professionally repaired?

Professional repair of error code B0138 at Airbag24 focuses on eliminating the actual hardware cause. After submitting the removed control unit, a comprehensive diagnosis is performed. Depending on the findings, contacts are cleaned, defective components on the circuit board are replaced, or software adjustments are made to restore full functionality.

The repair process at Airbag24 follows a proven procedure. First, the fault memory is read to verify error code B0138. Then, the circuit board is visually inspected for obvious damage such as corrosion or signs of water ingress.

The actual repair involves restoring the affected circuits. This may include cleaning contacts and connectors, replacing defective sensors or cables, or repairing the control unit’s circuit board itself. If analysis requires it, a software reset is also performed.

Following the repair, the fault memory is cleared, and the control unit undergoes a thorough functional test. This ensures that error code B0138 has been permanently resolved and that the device operates flawlessly again. All repairs come with 2 years' statutory liability for defects.