B1A78

Code

Generic B — Body

Front Passenger Occupant Detection Sensor Circuit Fault

Brand: Generic

Type: B — Body

Views: UK: 12 EN: 24 RU: 22

AI status

Completed

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Completed 100%

Page language: EN

Causes

Open or shorted wiring between sensor and airbag/occupant classification module
Corroded, loose, or damaged connector or pins at the sensor or module
Failed occupant detection sensor (mat/pad/cell) or sensor module
Water/moisture intrusion or contamination in the seat assembly
Damage caused by aftermarket seat covers, heavy objects, or repair work
Faulty occupant classification module or airbag control module

Symptoms

Passenger airbag OFF/ON indicator may be incorrect or illuminated
SRS/airbag warning lamp illuminated on instrument cluster
Passenger airbag disabled unexpectedly
Seat belt reminder or chime behavior changes for front passenger
DTC B1A78 stored in airbag/occupant classification module
Possible inconsistent occupant detection readings in live data

What to check

Read fault memory with a diagnostic scan tool and capture freeze-frame/live-data
Visually inspect seat area for water damage, foreign objects, or aftermarket covers
Inspect wiring and connectors under the front passenger seat for damage or loose pins
Check for related SRS/airbag codes that may indicate module or power issues
Perform a wiggle test on seat wiring while observing live-data for intermittent faults
Verify vehicle battery voltage and module ground connections before testing

Signal parameters

Typical reference supply: 5.0 V ± 0.5 V (varies by manufacturer) to occupant detection module/sensor
Sensor signal voltage range: typically 0.0–5.0 V depending on occupant load and sensor type
Continuity: harness should not be open; expected continuity depends on sensor design — consult OEM data
Resistance: occupant mat sensors vary widely (ohms to kilo-ohms) — use OEM specifications for target values
CAN/diagnostic communication: module should respond on diagnostic bus; no response indicates module/power/comm fault

Diagnostic algorithm

Safety first: follow SRS safety procedures. Disable SRS power per manufacturer instructions (battery disconnect or key-off interval) before unplugging connectors or performing repairs.
Retrieve and record DTCs and freeze-frame data with an OEM-level scan tool. Note related codes and live-data values for the passenger occupancy circuit.
Visually inspect the passenger seat area, underside of the seat, and seatbelt buckles for signs of moisture, contamination, or physical damage. Remove aftermarket seat covers if present.
With ignition ON (engine off), use the scan tool to monitor the occupant sensor live-data while performing a wiggle test on the seat wiring and connectors. Look for value changes or drop-outs that reproduce the fault.
Inspect connectors: unplug the passenger seat/occupant sensor connector(s) and inspect pins for corrosion, bent pins, or contamination. Reconnect securely and re-check for codes.
Measure supply and ground at the sensor connector: verify reference voltage present and good ground. If reference voltage missing, trace back to the module/fuse/ignition circuit.
Measure sensor output or resistance per OEM procedure. Compare with specification. If out-of-spec, replace the sensor/mat assembly.
If wiring and sensor check good but fault persists, inspect harness for internal breaks (backprobe or continuity test to module). Repair or replace damaged wiring/connector as needed.
If sensor and wiring are good, check occupant classification/airbag module power, grounds, and communication. Replace module only after confirming other components and following programming/calibration rules.
After repair, clear codes, perform any required module re-learning/calibration procedures, and re-check by recreating conditions to ensure the DTC does not return.

Likely causes

Broken or chafed harness in the seat wiring looms
Bent or corroded connector pins at the seat sensor connector
Seat mat sensor element physically damaged by foreign object or spill
Poor ground or missing reference voltage to sensor
Intermittent connection caused by seat movement (wiring routed across hinge)
Module powering or internal fault in occupant classification/airbag ECU

Fault status

⚠️ Status

Front passenger occupant detection sensor circuit fault — signal out of range, open/short, or intermittent (may disable passenger airbag).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2 hours

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