B1304 airbag control unit fault

What does error code B1304 mean?

Error code B1304 indicates a specific hardware fault in the “Up/Down” Topstack position sensor of the airbag system. The control unit receives an implausible signal because the sensor simultaneously reports both states—“Up” and “Down.” This results in a critical inconsistency.

Technically, this means that the internal logic of the airbag control unit is receiving a contradictory input from the position sensor. The Topstack sensor is a crucial component of the seat‑position monitoring systems, which are essential for the proper deployment of the airbags.

Due to this erroneous information, the system can no longer ensure safety and, as a protective measure, activates the airbag warning light. The fault primarily points to an issue with the sensor itself, its wiring, or the signal processing within the control unit.

How do you recognize error code B1304?

The main symptom of error code B1304 is a permanently illuminated airbag warning light on the instrument cluster. This light does not turn off after starting the vehicle, indicating an active, safety‑relevant fault in the system that requires immediate diagnosis.

When diagnosed with a suitable workshop tester, error code B1304 can be clearly read from the airbag control unit’s fault memory, confirming that the malfunction originates from the Topstack position sensor or its associated peripherals.

Typically, no other functional failures are observed in the vehicle. The issue is confined exclusively to the passive safety system, which remains non‑functional until the fault is resolved.

What causes B1304?

The cause of B1304 is almost always a hardware defect. First and foremost is an internal failure of the Topstack position sensor itself. Other common sources include electrical problems such as cable breaks, corroded connectors, or moisture damage in the wiring harness or control unit.

A sensor defect is one of the primary causes. Over time or due to internal damage, the component delivers incorrect, simultaneous signals to the control unit.

Electrical issues are also widespread. These include damaged cables—especially near the seat—poor contacts at connector joints, or corrosion caused by ingress of moisture. A short circuit can likewise produce conflicting signals.

Mechanical damage to seat‑adjacent components, such as the coil spring or seatbelt buckles, can indirectly lead to wiring problems. In rarer cases, the fault lies directly within the control unit—for example, a cold solder joint at the sensor connection pins or a damaged trace on the circuit board.

In which vehicles does error code B1304 occur?

Error code B1304 has been particularly frequently documented across various Mitsubishi models, including the L200, Pajero Sport, and Lancer. It may also appear in certain Infiniti models. At Airbag24, we repair airbag control units exhibiting this fault for a wide range of affected vehicles.

At Airbag24, we professionally restore error code B1304 for the following models and series:

Infiniti: QX55 P15 (2021–present)

Mitsubishi: L200 K74, L200 KJ (2015–2023), Lancer CZ, Lancer Evolution VI, Mirage CA, Pajero Sport K90 (1996–2008), Xpander Cross A05A

How does repair work at Airbag24?

Repair of error code B1304 focuses on eliminating the underlying hardware cause. After sending the removed control unit to Airbag24, it undergoes component‑level analysis; defective parts are repaired or replaced, and final system functionality is verified through testing.

The process begins with a visual inspection of all connectors and cables on the removed unit. Next, the control unit is opened to examine the circuit board. Technicians specifically check the solder joints at the sensor connections and the associated traces.

Defective components, such as connection pins or damaged ICs, are professionally replaced or re‑soldered at the board level. If crash data is stored in the memory, it is completely erased, restoring full functionality to the control unit.

Following successful repair, comprehensive functional tests are conducted to verify correct signal processing. The control unit is then returned to you ready for “plug-and-play” installation. We provide a 12‑month warranty on all repairs.