B120A

Code

Generic B — Body

Occupant Classification System (OCS) Malfunction

Brand: Generic

Type: B — Body

Views: UK: 17 EN: 11 RU: 10

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Page language: EN

Causes

Faulty occupant classification sensor(s) (seat mat, load cell, pressure sensor)
Damaged or corroded wiring/connectors between seat sensors and OCS module
Faulty OCS control module or module software error
Intermittent or low battery/charging system voltage during self-test
Water/moisture intrusion, seat foam contamination or foreign object under the sensor
Aftermarket seat covers, cushions or repairs that change sensor behavior

Symptoms

Airbag/OCS warning lamp illuminated on dash
Passenger airbag may be disabled when it should be enabled (or vice versa)
Passenger airbag status indicator shows incorrect state
Diagnostic trouble code B120A logged in body/airbag control module
Intermittent OCS warnings after seat movement, occupant shift or upholstery changes

What to check

Scan for stored and pending codes in SRS/BCM/OCS modules; record freeze frame and related codes
Check for additional body or network (CAN/LIN) fault codes that may affect OCS
Visually inspect seat base, underside and connector for water, corrosion, foreign objects or damage
Check battery voltage and charging system; perform tests with battery at nominal voltage
Verify there are no aftermarket seat covers or bulky objects on the seat
Perform seat belt buckle and seat position switch checks if applicable

Signal parameters

Sensor reference voltage (typical) — ~5.0 V reference (verify exact spec for vehicle)
Sensor signal range — varies by design; roughly 0.1–4.9 V proportional to load or resistance change
Ground continuity —
CAN or LIN message presence — OCS module broadcasts occupant status and sensor values on network (verify message IDs with OEM scan tool)
Self-test responses — OCS module internal diagnostics should report PASS/FAIL and error counters

Diagnostic algorithm

Read and record all SRS/BCM/OCS codes and related data with a capable scan tool. Note freeze-frame data and current occupant classification values.
Verify battery voltage is stable (12.4–14.8 V) during diagnostics. Recharge or charge if low and retest.
Inspect seat area: remove lower seat trim as needed; look for water, debris, damaged sensor mat, or aftermarket covers. Check connectors and pins for corrosion or damage.
Wiggle-test wiring harness while monitoring live sensor values/messages to reproduce fault or locate intermittent opens/shorts.
Measure reference voltage, signal voltage and ground at the sensor connector per OEM pinout. Compare to OEM spec. Check continuity from sensor connector to OCS module.
If sensor wiring and voltages are within spec, perform OCS module self-test and follow OEM-specific recalibration/relearn procedure for occupant classification after any repair or component replacement.
If a single sensor shows out-of-spec readings, replace that sensor (seat mat/load cell) and retest. If multiple sensors or communications are missing, focus on wiring harness and module replacement as required.
After repairs, clear codes and perform occupant detection validation with a qualified scan tool and test occupants per OEM procedure. Confirm no reoccurrence under normal conditions.
If fault persists and wiring is confirmed good, consider updating module software or replacing OCS control module following OEM procedures.

Likely causes

Connector corrosion or bent pins at seat module
Damaged harness from seat movement or seat rail chafing
Failed or shorted seat mat/load-cell sensor element
OCS module internal fault or failed calibration

Fault status

⚠️ Status

OCS Malfunction — occupant classification system detected a fault in sensors, wiring, or control module affecting passenger detection and airbag enable logic.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-4.0 hours