Code
B1121
HUMMER
B — Body
Driver Seat Belt Buckle Switch Circuit Malfunction
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring between driver buckle switch and BCM
- Faulty driver seat belt buckle switch (mechanical or electrical failure)
- Corroded, loose or damaged connector at the buckle or BCM
- Poor ground or reference voltage to the switch circuit
- Water intrusion or debris in buckle assembly
- BCM software/firmware glitch or BCM fault
Symptoms
- Driver seat belt reminder light/chime may not function correctly
- Seat belt buckle indicator (dash or BCM) shows incorrect status
- Occupant sensing / restraint warnings may appear or be disabled
- Possible MIL (if linked with other restraint diagnostics) or stored B-series DTC
- Intermittent fault that appears/disappears when seat moved
What to check
- Scan for stored/active codes and note freeze-frame/occurrence data
- Visually inspect buckle, connector, and under-seat wiring for damage, corrosion, or moisture
- Check connector pins for bent contacts and ensure proper mating
- Operate buckle while monitoring connector for intermittent faults (wiggle test)
- Refer to vehicle wiring diagram to identify connector and BCM pin locations
- Verify any manufacturer-required procedures for disabling SRS/airbag systems before service
Signal parameters
- With connector disconnected: buckle switch should show open (OL) or infinite Ω when unbuckled and continuity (low Ω) when buckled; closed continuity typically
- Back-probed at BCM or harness: switch signal will toggle between reference voltage (or pull-up voltage) and ground when buckled/unbuckled — exact voltage depends on vehicle (commonly 5 V or battery through pull-up).
- Voltage when closed: near 0 V if switch pulls to ground; when open: near reference/pull-up voltage (verify in service data)
- Intermittent faults may show fluctuating voltage or intermittent loss of continuity while operating seat
- Resistance on harness between buckle and BCM pins should be low (
Diagnostic algorithm
- Retrieve all DTCs and freeze-frame data. Note if other restraint or body codes are present.
- Visually inspect driver buckle, connector, and under-seat wiring for damage, corrosion, or foreign material. Repair obvious damage.
- With ignition ON, back-probe the buckle connector (or harness at a service connector) and observe signal while buckling/unbuckling. Compare to expected behavior from service data.
- Measure continuity between buckle switch terminals and BCM connector pins with harness disconnected; operate the buckle to confirm the circuit changes state (closed/open).
- If open or short is found in harness, trace and repair wiring (splice, replace harness section or connector). Protect and secure wiring to prevent future chafing.
- If wiring and connectors test good, replace the buckle switch assembly and retest.
- If buckle replacement does not resolve and wiring is good, consider BCM input fault — verify BCM ground and power supplies, check for related BCM codes, and consult service manual for BCM diagnostic procedures.
- After repair, clear codes, perform function check by operating buckle through range of motion and drive cycle as required to confirm the DTC does not return.
Likely causes
- Corroded/loose connector at the buckle or under-seat harness
- Faulty buckle switch (most common mechanical failure)
- Open or chafed wire in the under-seat harness (damage from seat movement)
- Intermittent connector contact due to debris or wear
Fault status
Status
B1121 — Driver Seat Belt Buckle Switch Circuit Malfunction: BCM has detected an open, short, intermittent or invalid signal from the driver seat belt buckle switch circuit.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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Repair manuals
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