B3129

Code

Generic B — Body

Occupant Detection Sensor Circuit Fault

Brand: Generic

Type: B — Body

Views: UK: 11 EN: 11 RU: 10

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Page language: EN

Causes

Open or short in occupant detection sensor wiring or connector
Corroded, loose or damaged seat harness connector
Failed occupant detection sensor (load cell/capacitive pad/module)
Poor or missing ground or reference voltage to the sensor
Intermittent connection due to seat movement, foam abrasion or pinched wire
Faulty restraint control module or software/calibration error

Symptoms

Airbag/airbag indicator or SRS warning lamp illuminated
Passenger airbag disabled or persistent passenger presence warnings
DTC present in restraint control module scan tool
Intermittent or consistent seat-occupancy reporting (false occupied/unoccupied)
Possible disablement of passenger airbag or seatbelt reminders

What to check

Scan vehicle with an OEM-capable scan tool; read and record freeze frame and live data for occupant sensor
Visually inspect seat connectors and wiring under the seat for damage, corrosion, or disconnected plugs
Check for water/moisture or debris in the seat cushion and connector area
Verify reference voltage (typically 5V) and ground at the sensor connector with ignition ON
Back-probe sensor signal wire and observe voltage change when occupant sits/stands
Wiggle test wiring while monitoring live data to look for intermittent faults

Signal parameters

Reference supply: typically 5 V (check OEM spec) present at sensor connector with IGN ON
Ground: near 0 V at sensor ground pin
Signal output: varies with occupancy — commonly low (≈0.2–0.6 V) when empty, rising into a higher range when loaded (≈1–4 V); consult OEM for exact thresholds
Sensor continuity/resistance: depends on sensor type (load-cell, strain gauge, or capacitive) — compare to OEM specification or the opposite seat sensor
CAN/Module messages: occupant presence and weight class values should update on the bus if sensor and module are OK

Diagnostic algorithm

Safety first: follow OEM SRS safety procedures. Disable the battery negative terminal and wait the recommended time before probing connectors or removing seats.
Connect a compatible scan tool, record DTC B3129 and any accompanying codes, note freeze-frame/live-data values related to the occupant sensor.
Perform a visual inspection of the seat cushion, sensor pad, connectors, and wiring harness for damage, pinched wires, or moisture. Repair obvious damage.
Re-enable power per OEM instructions (if needed) and back-probe the sensor connector. With ignition ON, verify reference voltage and ground at the connector pins match OEM values.
Observe the sensor signal while the seat is empty and while a known weight (e.g., a technician sitting or a calibrated weight per OEM procedure) is applied. Confirm the signal changes smoothly and crosses the occupancy threshold.
Perform a wiggle/tap test on harness and connector while monitoring live data to find intermittent connections. Check connector terminals for corrosion, bent pins, or poor contact.
If signal is open/shorted or out of range, isolate by disconnecting connector and measuring continuity from connector back to the restraint control module to locate wiring fault.
If wiring and connectors test good, measure sensor resistance/parameter per OEM spec or compare to the opposite seat sensor. Replace sensor if out of tolerance or non-responsive.
If sensor replacement does not clear the fault, check module grounds and module connector. Scan for any network errors (CAN) that could affect occupant data.
After repairs, clear codes and perform OEM-required occupant sensor calibration or zeroing procedure (many systems require a calibration step after sensor/service).
Retest: verify no reoccurrence of B3129 and confirm correct passenger presence reporting across expected weight ranges.