Code
B0749
HUMMER
B — Body
Occupant restraint sensor circuit — low voltage
AI status
Completed
Completed
100%
Causes
- Open or high resistance in sensor supply or signal wiring
- Damaged, corroded, or disconnected sensor connector (under/around seat)
- Faulty occupant restraint/occupancy sensor module or sensor mat
- Short to ground on the sensor signal or supply circuit
- Low reference supply voltage from SRS control module or battery supply issue
- Water intrusion or contamination in seat wiring harness or sensor
Symptoms
- SRS / Airbag warning lamp illuminated or flashing
- Passenger airbag status may be disabled or show incorrect availability
- Seat belt reminder or occupant classification messages incorrect
- Diagnostic trouble codes stored in the SRS / occupant restraint system
What to check
- Use a scan tool to read codes and view live data for the occupant sensor channel and related inputs
- Visually inspect seat and under-seat area for loose connectors, corrosion, damaged wiring, water intrusion, or aftermarket splices
- With ignition ON (engine off) backprobe sensor connector: check reference voltage (typically +5V) and ground continuity to chassis
- Measure signal voltage at the sensor connector while operating seat/placing weight to observe expected changes
- Check continuity and resistance of sensor wiring to the SRS module; wiggle harness while observing live data to reproduce fault
- Inspect and test battery voltage and main ground points — low battery/ground can affect sensor circuits
Signal parameters
- Typical reference supply: approx. 5.0 V (±0.25 V) at sensor connector with ignition ON
- Signal output: analog range 0.5–4.5 V depending on sensor/design (low-voltage fault indicated when signal < ~0.3 V or otherwise below expected range)
- Ground: close to 0 V; chassis-ground resistance should be low (
- Open-circuit: infinite resistance; short-to-ground: near 0 Ω; short-to-Vref: near reference voltage
Diagnostic algorithm
- Safety first: disable SRS power before disconnecting connectors or working on airbag components — disconnect battery negative and wait manufacturer-specified time, or follow vehicle service manual SRS disable procedure. If unsure, have a qualified technician perform SRS work.
- Read and record all SRS/occupant restraint codes and freeze frame data with a capable scan tool.
- Check battery voltage and main chassis grounds; correct any low voltage or poor ground conditions.
- With ignition ON (engine off) and using a backprobe, measure reference voltage and ground at the occupant sensor connector. If reference voltage is absent or low, trace power supply back to SRS module and fuses.
- Inspect the connector and wiring under the seat for corrosion, water damage, pinched or cut wires. Repair or replace damaged sections and ensure connector pins are clean and secure.
- Perform continuity and resistance checks on the signal, reference, and ground circuits between the sensor connector and the SRS control module. Repair any opens/high resistance.
- If wiring and supply check good, bench-test or replace the occupant sensor per OEM procedure. After replacement, reconnect, clear codes, and perform any required occupant classification/learn or calibration routines with a scan tool.
- Re-scan the system and verify no DTC returns and that SRS lamp and passenger airbag status behave correctly.
- If fault persists after repairs and known-good sensor, consider SRS control module fault or intermittent wiring; consult wiring diagrams and perform module-level diagnostics or replacement as last resort.
Likely causes
- Corroded or disconnected connector at occupant sensor under the seat
- Broken or pinched wiring in the seat harness (common near seat track)
- Failed occupant classification sensor/mat
- Low 5V reference from SRS module or poor ground connection
Fault status
Status
DTC B0749 — Occupant restraint sensor circuit: low voltage detected. SRS warning lamp illuminated; passenger restraint availability may be affected. Repair wiring/connector/sensor as needed and clear code.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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138
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