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Home / DTC / B129B — Passenger Presence Sensor Circuit Fault

B129B — Passenger Presence Sensor Circuit Fault

Detailed page for trouble code B129B.

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B129B

Generic B — Body

Passenger Presence Sensor Circuit Fault

Brand: Generic
Type: B — Body
Views: UK: 4 EN: 9 RU: 4
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Page language: EN

Causes

  • Damaged or disconnected passenger presence mat/sensor
  • Corroded, loose, or damaged wiring or connector (open/short/intermittent)
  • Water intrusion or contamination in the seat cushion/mat
  • Faulty occupant detection module or SRS control unit
  • Aftermarket seat cover, seat heater, or modification affecting sensor readings
  • Poor ground or reference voltage to the sensor

Symptoms

  • SRS / airbag warning light illuminated
  • Passenger airbag may be disabled when seat is occupied or enabled when empty
  • Incorrect passenger presence indicator or seatbelt reminder behavior
  • Occupant detection status shows invalid or fluctuating values in scan tool
  • Possible chime/warning related to passenger seat

What to check

  • Read and record stored DTCs and freeze-frame data with a compatible scan tool
  • Follow SRS safety procedures: disable battery and wait specified time before disconnecting components
  • Visually inspect seat bottom, connectors, and harness for damage, corrosion, or moisture
  • Wiggle test wiring and connectors while observing live sensor data for intermittent faults
  • Inspect for aftermarket seat covers, cushions, or heaters that may affect sensor
  • Check for separate related codes in airbag/occupant classification modules

Signal parameters

  • Supply/reference voltage (vehicle dependent): typically 5 V or 12 V reference to sensor circuit — must be stable
  • Signal output: voltage change, resistance change, or CAN/lin message indicating occupancy state
  • Resistance behavior: mat typically shows low resistance (kΩ range) when occupied and high resistance/infinite when empty — exact values vary by design
  • Expected stable, non-noisy waveform or steady occupancy status on live-data; frequent drops or noise indicate wiring/sensor issues
  • CAN/Module data: Occupancy state codes (0 = empty, 1 = occupied, etc.) — verify consistent messaging

Diagnostic algorithm

  1. Verify DTC B129B with a scan tool and note freeze-frame data and any pending codes. Check for related SRS codes.
  2. Review manufacturer-specific service information and TSBs for occupant classification/seat sensor procedures and known issues.
  3. Follow SRS safety procedures: turn ignition off, disconnect negative battery terminal, and wait manufacturer-specified time before accessing connectors.
  4. Visually inspect the passenger seat cushion, sensor mat, connectors, and harness for physical damage, water, corrosion, or signs of repair or aftermarket items.
  5. Reconnect battery (if disconnected for checks) and with a scan tool monitor live occupant sensor data. Perform a wiggle test on seat harness and connectors to identify intermittent changes.
  6. Measure reference voltage and ground at the occupant sensor connector with key on (per vehicle procedure). Verify proper supply and ground continuity back to module.
  7. Measure signal output or resistance across sensor mat pads per service manual. Compare to expected ranges or observe a clear change when a known weight is placed on the seat.
  8. If wiring or connector fault suspected, perform connector pin backprobe checks and continuity tests to module; repair any damaged wiring or corroded terminals.
  9. If moisture/contamination is found, dry/clean or replace the sensor mat as required. Replace seat covers that interfere with sensor operation.
  10. If sensor mat and wiring test good but fault persists, consider replacing the occupant detection module or SRS control module per manufacturer guidance and reprogram/calibrate as required.
  11. After repairs, clear codes, perform occupant classification calibration or system learning (if required), and retest for correct function under different occupant conditions.

Likely causes

  • Connector corrosion or pin damage at the seat sensor harness
  • Open or shorted signal wire between sensor mat and module
  • Sensor mat failure (broken traces or internal short due to moisture)
  • Seat heater harness interference or short affecting sensor circuit
  • Faulty occupant classification module or SRS control wiring

Fault status

⚠️ Status
Passenger Presence Sensor Circuit Fault — the occupant detection circuit is reporting an open/short/erratic signal; system may disable or misclassify passenger airbag until fault is resolved.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-2.5 hours

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