Code
B2791
Generic
B — Body
Start Inhibit Request Signal Fault
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UK: 1
EN: 4
RU: 0
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in the start-inhibit request circuit
- Poor or corroded connector or ground at BCM/immobilizer/ignition switch
- Faulty immobilizer or body control module (BCM) output driver
- Failed starter inhibit/enable relay or starter control device
- Blown fuse or poor battery/low system voltage causing invalid signal
- Intermittent connector contact or water intrusion
Symptoms
- Engine will not crank or start when expected (no-crank)
- Starter relay does not energize when key/button is operated
- Security or immobilizer warning lamp illuminated or flashing
- Intermittent no-start conditions that may clear with reconnection or vibration
- CAN/communication warnings or mismatch between modules reported by scan tool
What to check
- Read and record freeze-frame and related stored codes with a scan tool; note module reports and module voltages
- Verify battery voltage (resting and cranking) — low battery can cause false faults
- Inspect fuses and fusible links for ignition/immobilizer/BCM circuits
- Visually inspect connectors and wiring at ignition switch, immobilizer, BCM and starter relay for corrosion, damage, pin push-out or water ingress
- Check for presence/behavior of security/immobilizer lamp on dash during key-on
- Attempt a start while monitoring the start-inhibit signal with a multimeter/oscilloscope at the module connector (backprobe)
Signal parameters
- Logic voltage levels (typical, design dependent): Low:
- If signal is active-low or active-high depends on manufacturer — confirm with service data before repair
- Open-circuit: infinite resistance between signal conductor and module output; short to battery: ~0 Ω to +12V; short to ground: ~0 Ω to chassis ground
- If PWM is used: steady DC when inactive; if PWM present, verify frequency and duty cycle with scope per OEM specs
- Connector resistance: expect 2–5 Ω) may indicate corrosion or partial break
Diagnostic algorithm
- Confirm the code and capture freeze-frame / related network data with a scan tool
- Verify battery health: measure resting voltage and voltage while cranking; recharge or replace if low
- Visually inspect wiring and connectors at the immobilizer, BCM and ignition switch for damage, corrosion, loose pins or water intrusion
- Backprobe the start-inhibit request pin at the controlling module with key ON and attempt to start; record voltage behavior (stays low/high, fluctuates, goes open)
- Check fuses and relays related to ignition/immobilizer and starter. Swap the relay with a known-good same-type relay if available
- Perform continuity and short-to-power/ground checks on the signal circuit from module to starter relay/enable device with the battery disconnected
- Inspect and test clutch/neutral safety switch (if applicable) and starter relay coil circuit to ensure the enable path is intact
- If wiring and external devices test good, test or replace the immobilizer/BCM output driver per manufacturer procedure or perform module reflash/initialization if software update or corruption is suspected
- Clear codes after repairs and attempt multiple start cycles; monitor for recurrence and perform road/test cycles as required
Likely causes
- Broken wire between ignition switch/immobilizer and BCM
- Corroded connector at immobilizer or BCM pins for start-inhibit output
- Faulty immobilizer module not asserting correct logic level
- Stuck or failed starter inhibit relay contact or coil
- Blown or high-resistance fuse in ignition/start circuit
- Low battery voltage or poor chassis ground affecting logic thresholds
Fault status
Status
Start Inhibit Request Signal Fault — start enable/request circuit out of expected range or not present; may prevent starter operation.
Repair difficulty: Medium
Diagnostic time: 1.0 - 2.5 hours
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