B1120

Code

Generic B — Body

Occupant Classification System (OCS) Sensor Circuit Fault

Brand: Generic

Type: B — Body

Views: UK: 12 EN: 30 RU: 13

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open, short to ground, or short to battery in OCS sensor wiring
Loose, corroded or damaged seat connector or terminals
Failed OCS (seat weight/occupancy) sensor pad/module
Poor or missing ground or reference voltage to the sensor
Water intrusion, contamination or mechanical damage in the seat
Faulty occupant classification or SRS control module

Symptoms

Airbag or SRS warning lamp illuminated
Passenger airbag disabled or incorrectly enabled
Passenger airbag status (ON/OFF) message inconsistent with occupancy
Stored or intermittent DTC B1120 (may be active or pending)
Loss of occupant classification data or incorrect seat occupancy readings

What to check

Read DTCs and freeze frame data with a capable scan tool; note related SRS/OCS codes
Verify whether the passenger airbag indicator shows ON/OFF and if it matches occupant presence
Visually inspect seat cushion, underside, connectors and wiring harness for damage, corrosion or contamination
Follow manufacturer procedure to disable SRS (battery disconnect or specified service mode) before disconnecting connectors or removing the seat
Backprobe the OCS connector with key on (or per manual) and check reference voltage and ground
Wiggle test wiring and connectors while monitoring live data or voltage to locate intermittent faults

Signal parameters

Sensor reference voltage: typically ≈5.0 V (nominal) — acceptable range often ~4.5–5.5 V (verify with OEM spec)
Sensor ground: continuity to chassis ground typically
Sensor output: variable analog or digital signal—open-circuit or output voltage commonly 0.5–4.5 V depending on occupant/load (compare to OEM live-data values)
Insulation/leakage: sensor circuit to chassis should measure very high resistance (>>1 MΩ) with occupant sensor disconnected
Short detection: diagnostic expects no short to battery or ground; short to Vb ~0 Ω or to GND ~0 Ω indicates fault
Wake/communications: some systems use low-frequency polling or pulse trains — expected frequency/pattern should be checked with OEM data or oscilloscope

Diagnostic algorithm

Retrieve DTC(s) and live data with a scan tool. Note whether B1120 is current or stored and check for related codes (SRS/OCS).
Review freeze frame and live occupant classification values (raw sensor channels) while occupant present/absent to confirm sensor behavior.
Visually inspect the seat cushion, sensor mat, connectors under the seat, and wiring for damage, loose terminals, corrosion or signs of liquid exposure.
Follow the vehicle manufacturer’s SRS disable procedure before disconnecting or handling seat/airbag components.
With connector accessible and key ON per OEM procedure, backprobe the sensor connector: verify reference voltage (≈5V), sensor ground continuity, and sensor output. Compare to OEM expected ranges.
With an ohmmeter (key off), check continuity of the wiring from the sensor connector back to the SRS/OCS module and check for shorts to chassis or battery. Wiggle the harness and connectors to reproduce intermittent faults.
If sensor output is abnormal, disconnect the sensor and verify that the module reports an open or changed status; reconnect and confirm change. This helps isolate sensor vs. module/wiring.
Inspect and test connector pins for corrosion, bent pins, or poor crimps. Repair or replace damaged connectors and re-test.
If wiring and connectors check good, replace the suspect OCS sensor pad and re-test. Some systems require recalibration/zeroing after sensor replacement—perform per manufacturer procedure.
If symptoms persist after sensor replacement and wiring verified, consider SRS/OCS control module fault; consult OEM diagnostics for module testing or replacement.
After repair, clear DTCs and verify the code does not return and that passenger airbag/OCS behavior is correct during function checks.

Likely causes

Broken or chafed wire under the seat (pinch point/wear)
Connector corrosion from moisture or spilled liquids
Failed sensor element in the seat cushion
Intermittent connection at seat track/plug due to movement
Module internal fault or corrupted calibration data
Improperly routed wiring after previous seat removal

Fault status

⚠️ Status

Circuit fault detected in Occupant Classification System sensor(s). The SRS/OCS module has stored a malfunction (open/short/intermittent) that may affect passenger airbag enable/disable logic and will typically illuminate the airbag warning lamp.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours