B109A

Code

LAND ROVER B — Body

Passenger occupant detection sensor circuit fault

Brand: LAND ROVER

Type: B — Body

Views: UK: 18 EN: 31 RU: 25

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Page language: EN

Causes

Open or shorted wiring/harness to the passenger seat occupancy sensor
Corroded, loose or damaged connector/pins under the seat
Failed or degraded seat occupancy sensor/mat
Water ingress or foreign contamination in the seat cushion
Poor ground or supply (reference) voltage to the sensor
Faulty restraint control module or software fault

Symptoms

SRS/airbag warning lamp illuminated
Passenger airbag enable/disable message incorrect or not changing with occupancy
Seatbelt reminder behavior abnormal for passenger seat
Vehicle stores B109A and may store related SRS occupant-sensor codes
Possible inability to register passenger presence

What to check

Read and record all stored and pending SRS codes with a compatible scan tool; note freeze-frame data
Inspect seat base and carpet area for water, corrosion or damage
Visually inspect seat harness connectors, pins and routing for chafe or deformation
Check for proper connector engagement and locking tabs
Monitor occupant sensor live data/values while changing seat occupancy (use scan tool)
Wiggle harness and seat while watching live data for intermittent faults

Signal parameters

Reference supply: typically ~5 V (varies by vehicle) present at sensor reference pin with ignition ON
Signal output: varies with occupancy (voltage or resistance change); should change consistently when weight applied/removed
Open-circuit: infinite resistance or no signal; Short-to-ground: near 0 V; Short-to-battery: near battery voltage
Expected continuity: harness pins to module should be continuous (low ohms) — open or high resistance indicates break
Scan-tool live values: sensor status parameter should toggle between 'occupied' and 'unoccupied' when tested

Diagnostic algorithm

Read and record codes and freeze-frame data with a diagnostic tool that supports the vehicle's SRS system.
Check for related occupant or SRS codes; clear codes and perform a short test drive or occupancy test to see if B109A returns.
Observe occupant-sensor live data while applying/removing weight from the passenger seat. Note whether values change and whether they are within expected bounds.
Visually inspect the passenger seat, connectors under the seat, carpet access points and wiring harness for damage, corrosion or water ingress. Repair any obvious physical damage.
With ignition OFF and battery isolated per manufacturer SRS safety procedure, disconnect the seat connector and inspect pins for corrosion/bending. Reconnect securely.
With correct safety precautions (battery reconnected per manufacturer guidance), measure reference voltage and signal voltage at the sensor connector with a multimeter while monitoring live data. Compare to expected signal behavior (open/short/steady).
Check continuity and for short-to-ground or short-to-B+ between the sensor connector and the RCM connector (battery disconnected). Repair any short/open in wiring harness.
If wiring and connectors test good but signal is out-of-spec or no change with occupancy, replace the passenger seat occupant sensor/mat per manufacturer procedure.
After repair or replacement, perform required seat/occupant sensor calibration or SRS self-test with the manufacturer scan tool. Clear codes and verify that B109A does not return.
If fault remains after harness and sensor replacement, consider restraint control module fault and refer to manufacturer diagnostics/replacement procedures.

Likely causes

Damaged connector, bent pin, or corrosion at seat harness connector
Open or short in seat sensor wiring (common when seat slides/moves)
Failed occupancy mat/sensor element under seat cushion
Intermittent connection caused by seat movement/wiring chafe

Fault status

⚠️ Status

Passenger occupant detection sensor circuit fault — the restraint control module has detected an open, short, intermittent or out-of-range signal from the passenger seat occupancy sensor. Inspect connectors/wiring, test sensor and perform calibration after repair.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours

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