Code Definition

B0121 (26) — RHF seat belt buckle switch circuit malfunction

System: Body  |  Type: Generic (OBD-II Standard)  |  Make: Toyota

About B0121

B0121 is a generic (obd-ii standard) body diagnostic trouble code recorded for Toyota vehicles. It belongs to the B-prefix family of codes, which covers body, interior, airbag, or comfort-electronics faults.

When B0121 is stored, the module has detected an electrical-circuit fault in the wiring, connector, or driver associated with the named component.

Scanning & clearing: Body (B) codes are often stored in the body control module, airbag module, or climate module. A basic OBD-II tool usually will not read body codes — an enhanced or OEM-level scan tool is typically required.

Because B0121 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The airbag control module, called the Center Airbag Sensor Assembly or SRS ECU on Toyota, monitors the right hand front seat belt buckle switch to know whether the front passenger belt is fastened. DTC B0121 sub code 26 sets when the module reads an abnormal, out of range, or implausible signal from that buckle switch circuit, such as an open, a short, or a state that never changes when the belt is buckled. The buckle switch is a simple contact inside the belt receiver that changes resistance or voltage between the fastened and unfastened positions, and the SRS ECU uses this input for the belt...

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