B1602 airbag control unit fault

What does error code B1602 mean in the airbag control unit?

Error code B1602 is a diagnostic trouble code (DTC) indicating a communication issue between the ignition key transponder and the vehicle’s control unit. Its official description is “Invalid key code format received from the ignition key transponder,” pointing to a fault in the Passive Anti-Theft System (PATS), i.e., the immobilizer.

Although this error primarily affects the immobilizer, it can also be stored in the airbag control unit (SRS module). This occurs in vehicle architectures where the airbag control unit plays a central role in on-board network communication and participates in validating safety‑related signals, such as those from the ignition key.

Technically, the control unit receives a signal from the key whose data format does not match the expected specifications. As a result, the system cannot recognize the key as authorized, leading to the storage of the error code and potentially activating the immobilizer.

How do you recognize error B1602?

The most obvious symptom of error B1602 is the persistent illumination of the airbag warning light (SRS light) in the instrument cluster. Additionally, issues with the immobilizer may arise, manifesting as difficulty starting the engine or a flashing immobilizer symbol. The vehicle fails to properly recognize the key.

The airbag warning light remains illuminated after the engine starts and does not extinguish as usual, signaling an active, safety‑relevant fault within the SRS system that could impair airbag functionality.

In some cases, the engine may fail to start altogether. Since the error code indicates invalid key authentication, the immobilizer can remain active, blocking ignition or fuel delivery to prevent theft.

What causes error code B1602?

The causes of B1602 are predominantly related to the immobilizer system. First and foremost is often a defective or improperly functioning transponder chip in the ignition key. Other common sources include a damaged antenna coil around the ignition lock, problems in wiring or connectors, as well as incorrect coding or an internal software fault within the control unit itself.

A frequent trigger is the ignition key itself. The transponder chip inside the key can be damaged by drops or simply fail due to aging. Using a key that has not been properly programmed or is mismatched can also lead to this error.

The antenna—a coil surrounding the ignition lock that receives the transponder signal—may also be faulty. A broken cable or corroded contacts in the wiring leading to the control unit can disrupt communication and generate the error code.

Finally, the fault may lie within the control unit itself. Incorrect programming, for example after component replacement, or a rare software bug can cause valid key codes to be mistakenly interpreted as invalid.

In which vehicles does error B1602 occur?

Error code B1602 can appear across various manufacturers where the airbag control unit is deeply integrated into the immobilizer communication system. At Airbag24, we specialize in repairing control units exhibiting this fault pattern, particularly for models from Alfa Romeo, Land Rover, and Toyota. The precise interpretation of the error may vary depending on the vehicle generation.

Based on data from our workshop experience, this error code appears in the following models:

Alfa Romeo: Especially in the 90 series (Type 162).

Land Rover: Affected are the first-generation Freelander (1997–2003) as well as its subsequent facelift model (2003–2006).

Toyota: The fault is known in the 4Runner TRD Pro and the Land Cruiser Prado.

How is error B1602 repaired?

A professional repair of error B1602 focuses on eliminating the root cause within the control unit. Upon submission, Airbag24 conducts a detailed analysis. Depending on the diagnosis, a software reset is performed, programming is corrected, or defective hardware components on the circuit board are expertly refurbished. A final test ensures full functionality.

The process begins at our local workshop with an inspection of external components such as the ignition key and antenna coil. If these can be ruled out as the source of the problem, the airbag control unit is removed and sent to us for repair.

At Airbag24, the unit is first tested on specialized test benches to precisely identify the fault. If the issue is software-related, it can often be resolved by resetting or reprogramming the device’s software. In the case of a hardware defect—such as in memory chips or the communication interface—the affected components are replaced by our technicians.

Following successful repair, each control unit undergoes a thorough quality inspection to ensure flawless operation. Repair offers a cost-effective and sustainable alternative to expensive new parts and typically does not require re-coding in the vehicle. We provide a 12-month warranty on all repairs.