B3120

Code

Generic B — Body

Passenger airbag occupant detection sensor fault

Brand: Generic

Type: B — Body

Views: UK: 8 EN: 10 RU: 10

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Damaged or disconnected wiring harness to the passenger occupant detection sensor (seat mat)
Corroded or loose connector at the sensor or airbag control module
Failed occupant classification sensor/seat mat (internal open/short or sensor element failed)
Water intrusion or contamination in the seat cushion or sensor mat
Incorrect or missing calibration after seat or sensor replacement
Faulty airbag control module or internal circuit fault

SRS/airbag warning lamp illuminated on dash
Passenger airbag OFF/ON indicator behaves incorrectly (on when no occupant, off when occupant present)
Passenger airbag disabled (no deployment) or system places module into a reduced/limp SRS mode
Intermittent fault that may appear/disappear with seat position or after jostling the seat
Related DTCs present for passenger airbag circuits or occupant classification

Use a diagnostic scan tool to read B3120 and any related SRS codes and freeze frame data
Verify passenger airbag status indicator (ON/OFF) on instrument panel with known occupant weight or test tool
Visually inspect connectors under passenger seat for dirt, corrosion, bent pins, moisture, or foreign objects
Perform a wiggle test of the seat wiring harness while monitoring live data/continuity for intermittent faults
Check for water/moisture staining or mechanical damage to the seat cushion and sensor mat
Confirm proper battery voltage and ground integrity before testing (key ON engine OFF for SRS checks)

Reference supply: typically a stable reference voltage provided by SRS module (commonly ~5 V) — verify at sensor connector
Signal: voltage or multiplexed data that varies with occupant load (expected to change when weight applied)
Sensor resistance/impedance: occupant mat should show measurable change when load applied; open or infinite indicates break
Communication: some systems use LIN or dedicated sensor serial links; others use analog signals to the SRS module
Fault indicators: short to ground, short to battery, or open circuit conditions will be flagged by the airbag control unit

Read all SRS codes with a capable scan tool; record freeze frame and related codes. Do not assume B3120 is the only fault.
Review TSBs or manufacturer service information for occupant classification specifics and calibration procedure.
With ignition ON (engine OFF), check sensor connector pins for reference voltage, ground continuity, and signal changes while applying a known load to the seat.
Visually inspect and test continuity of wiring from seat connector to body harness and to the airbag control module; repair any damaged wires.
Remove seat trim as required (follow SRS safety procedures—disconnect battery and wait manufacturer-specified time before servicing) and inspect/remove the occupant detection mat to check for physical damage or moisture.
If connector and wiring are good but signal is out of expected range or open, replace the occupant detection sensor/mat assembly.
After replacement or repair, reconnect battery, use scan tool to clear codes, perform occupant classifier calibration/initialization per manufacturer procedure, and confirm absence of faults and correct passenger airbag status.
If fault persists after sensor/mat replacement and correct calibration, evaluate or replace the airbag control module and verify software updates or reflash procedures per manufacturer.