B2851

Code

Generic B — Body

Seat Occupant Detection System Circuit Malfunction

Brand: Generic

Type: B — Body

Views: UK: 8 EN: 17 RU: 14

AI status

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Page language: EN

Causes

Damaged or disconnected seat occupant detection (SOD) mat or sensor
Open, shorted, corroded or pinched wiring or connector in SOD circuit
Poor or lost ground or supply voltage to the sensor/module
Faulty seat occupant detection module / occupant classification ECU
Water intrusion or contamination in the seat, mat or connector
Aftermarket seat cover or modification interfering with sensor

Symptoms

Passenger airbag ENABLE/DISABLE logic incorrect or warning light on
Restraint / airbag warning lamp illuminated on dash
Seat belt or passenger presence indicator incorrect
Diagnostic trouble code B2851 stored in SRS/Body control module
Possible inability to complete occupant classification self-test or calibration

What to check

Use a scan tool to read DTCs and live data for the seat occupant detection circuit; note freeze-frame and any related codes
Visually inspect seat, connectors, wiring harness and seat rail area for damage, corrosion or moisture
Check connectors are fully seated and locking tabs intact; disconnect and inspect pins for corrosion or bent pins
Perform a wiggle test of the wiring while monitoring live data or DTC status
Verify required power (typically fused 12V or ignition-switched) and ground at the occupant sensor/module
Inspect for aftermarket seat covers, seat heaters or repairs that could interfere with the mat

Signal parameters

Supply voltage: typical reference/power to sensor ~ 5 V or 12 V depending on design; verify against manufacturer data
Signal voltage: occupant sensor output may vary between ~0–5 V (or a resistance change) depending on load; should change when a known weight is placed on the seat
Continuity: harness continuity to module pins should be low/near 0 Ω for power/ground and within manufacturer spec for signal/resistive elements
Insulation: no short to chassis; measured resistance to chassis should be very high (>>1 MΩ)
Live-data behavior: signal should be stable and change smoothly when weight is applied/removed; intermittent spikes or open-circuit are indicative of a wiring or sensor fault

Diagnostic algorithm

Read and record DTCs and freeze-frame data with a capable scan tool. Note related SRS/Body codes.
Perform a visual inspection of the passenger seat, trim, connectors under the seat, and wiring along the seat rails and B-pillar for damage or contamination.
With ignition on, verify supply voltage and ground at the occupant sensor connector and at the occupant classification module connector. Compare to vehicle reference values.
Monitor live-data on the occupant detection channel while performing a wiggle test on the harness and connector; watch for changes or dropouts that reproduce the fault.
Measure resistance or signal at the sensor element (seat mat) per manufacturer procedure. If the mat is a resistive or capacitive sensor, check for expected change when applying known weight.
If wiring is suspect, perform continuity and short-to-ground/short-to-V tests between sensor and module pins. Repair any open/short/corroded wiring or connectors.
If wiring and connectors are good but sensor mat readings are out-of-spec, remove seat trim as needed and inspect/replace the sensor mat.
If mat and harness are good, consider replacement or bench-testing of occupant classification module per service manual.
After repairs, clear codes and perform any required occupant classification/airbag calibration or enabling procedure specified by manufacturer. Re-test to confirm code does not return.
If intermittent or not reproducible, consult vehicle wiring diagrams and manufacturer service information for more detailed tests and potential software updates.

Likely causes

Connector corrosion or loose connector at the seat mat or module
Damaged sensor mat (tear, crush, moisture) under the seat trim
Open/short in harness where it passes through the seat or B-pillar
Faulty occupant classification module (less likely than wiring/mat)
Interference from aftermarket seat covers or seat heater faults

Fault status

⚠️ Status

Seat Occupant Detection System Circuit Malfunction — sensor circuit fault detected. Passenger occupant classification may be unavailable or incorrect; airbag enable/disable may be affected.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-2.5 hours