Code
B1520
FIAT
B — Body
Bonnet switch circuit
Views:
UK: 5
EN: 14
RU: 6
AI status
Completed
Completed
100%
Causes
- Faulty bonnet (hood) switch
- Damaged, corroded or disconnected wiring/connector between switch and body control module (BCM)
- Poor ground or corroded connector pins
- Short to battery supply or short to ground in the circuit
- Blown fuse or related power/ignition supply fault
- Water ingress or contamination of switch/connector
Symptoms
- Bonnet/hood open warning lamp or message on instrument panel
- Interior or engine bay lights remaining on after closing bonnet
- Vehicle alarm/immobiliser won't arm or false alarm triggers
- Cruise control or other systems inhibited by hood-open status
- Intermittent bonnet open/close status or sporadic warnings
What to check
- Read freeze frame and current DTCs with a diagnostic scanner; note when code set (ignition status, engine running, bonnet position)
- Visual inspection of bonnet switch, latch area, wiring harness and connector for corrosion, damage or water entry
- Check fuses related to body/BCM circuits
- Operate bonnet switch manually while observing instrument cluster status and scan tool data
- Backprobe switch connector and measure voltages with ignition ON and bonnet open/closed
- Check continuity between switch and BCM connector, and continuity to ground where applicable
Signal parameters
- Switch closed (bonnet closed): circuit typically pulled to ground — near 0 V at BCM input (0–0.5 V)
- Switch open (bonnet open): input pulled to supply via pull-up — near battery voltage (~9–14 V) depending on vehicle
- Closed-switch resistance: typically very low (
- Open-switch resistance: high/OL when switch is open
- Typical circuit current: very low (milliamps) — main checks are voltage and continuity rather than high current
Diagnostic algorithm
- Use a scan tool to read B1520 and any related codes; record freeze-frame data and current bonnet status. 2. Visually inspect bonnet latch, switch and connector for corrosion, debris, water, or mechanical damage. Clean and reseat connector. 3. With ignition ON, backprobe the switch connector and measure voltage with bonnet open and closed; compare to expected signal parameters. 4. Check continuity from the switch terminal to the BCM input pin; inspect for intermittent opens by flexing the harness at the hinge and connector. 5. Check for shorts: measure resistance to battery positive and to ground with switch disconnected to identify a short to supply or ground. 6. Repair or replace damaged wiring, corroded pins or the switch as needed. After repairs, clear codes and verify that the fault does not return during several open/close cycles. 7. If wiring and switch verify OK and problem persists, inspect BCM input circuit for internal fault and consider module diagnosis or replacement as a last resort (refer to manufacturer procedures).
Likely causes
- Corroded or water-damaged bonnet switch or connector
- Broken or chafed wire in the harness at hinge area
- Connector not fully seated or pins bent
- Switch failed mechanically (stuck or not making contact)
Fault status
Status
Bonnet switch circuit malfunction — open/short/intermittent signal detected. Check bonnet switch, wiring, connectors and BCM input.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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