B1520

What Does Code B1520 Mean?

Diagnostic Trouble Code B1520 indicates an “Hood Switch Circuit Open” condition. This code is typically set by the Body Control Module (BCM) or a dedicated Security Control Module (SCM), which monitors the electrical state of the hood switch. The hood switch is a critical input for various vehicle systems, predominantly the anti-theft alarm, remote start functionality, and sometimes even auxiliary lighting control. When the BCM/SCM detects a continuous open circuit in the hood switch’s electrical path, contrary to the expected state (e.g., hood closed but circuit remains open), it registers this fault. An open circuit implies a break in electrical continuity, meaning the module is not receiving the expected signal or voltage from the switch, leading it to believe the hood’s state is unknown or perpetually open, even if physically closed.

Common Symptoms

• Inoperative or Erratic Anti-Theft System: The vehicle alarm may fail to arm, disarm, or may trigger spontaneously.
• Disabled Remote Start Function: Most vehicles inhibit remote start if the hood is detected as open for safety and security reasons.
• Illuminated Security or Malfunction Indicator Lamp (MIL): While not a powertrain code, some body-related faults can trigger a security light or a general warning lamp on the dashboard.
• Diagnostic Tool Reports Hood Ajar: A scan tool accessing BCM/SCM live data will show the hood status as “Open” or “Ajar,” even when it is physically closed.
• (Less common) Persistent Headlight Illumination: On some models, the headlamps may remain on if the system believes the hood is open.

What Causes the Code B1520?

• Faulty Hood Switch: The most common cause is a defective mechanical or magnetic hood switch. This can involve internal electrical failure, corrosion of contacts, or physical damage preventing proper actuation.
• Damaged Wiring Harness: An open circuit in the wiring connecting the hood switch to the BCM/SCM. This can be due to cuts, chafing, corrosion, or rodent damage within the engine bay, where the wiring is exposed to harsh conditions.
• Corroded or Loose Electrical Connectors: Poor electrical contact at either the hood switch connector or the BCM/SCM connector due to corrosion, bent pins, or insufficient terminal tension.
• Incorrect Hood Switch Adjustment: The hood switch plunger or sensor mechanism may be misaligned or incorrectly adjusted, failing to make proper contact or activate when the hood is fully closed.
• Faulty Body Control Module (BCM) or Security Control Module (SCM): Although less frequent, a defective input circuit within the BCM/SCM itself could erroneously report an open circuit for the hood switch.

How to Diagnose and Troubleshoot

Diagnosis of B1520 requires a methodical approach, utilizing visual inspection, a digital multimeter (DMM), and an advanced diagnostic scan tool:

1. Visual Inspection:
   • Locate the hood switch, typically positioned near the hood latch mechanism or on the inner fender structure.
   • Inspect the switch for any visible signs of physical damage, corrosion, dirt, or debris that might impede its operation.
   • Carefully trace the wiring harness from the hood switch back toward the firewall and into the cabin. Look for any signs of fraying, cuts, pinch points, exposed wires, or rodent damage.
   • Examine the electrical connectors at the hood switch and, if accessible, at the BCM/SCM for corrosion, bent pins, or loose connections. Ensure the hood itself is properly aligned and latches securely.
2. OBD-II Scan Tool Live Data:
   • Connect a diagnostic scan tool capable of communicating with the vehicle’s body control modules (BCM/SCM).
   • Navigate to the “Body” or “Security” module’s live data parameters.
   • Locate the parameter for “Hood Status,” “Hood Ajar Switch,” or similar.
   • Observe the displayed status while manually opening and closing the hood. A properly functioning circuit should show a clear state change (e.g., from “Closed” to “Open”) as the hood is manipulated. If the status remains persistently “Open” regardless of the hood’s physical position, it confirms an issue within the hood switch circuit.
3. Digital Multimeter (DMM) Testing:
   • Hood Switch Continuity Test:
     • Disconnect the electrical connector from the hood switch.
     • Set the DMM to resistance (ohms) or continuity mode.
     • Actuate the hood switch manually (simulate hood closed) and test for continuity across its terminals. For a normally open (NO) switch, it should show continuity (near 0 ohms) when depressed and open (infinite resistance) when released. For a normally closed (NC) switch, the opposite applies. Refer to the vehicle’s wiring diagram for the specific switch type. If the switch does not exhibit the correct continuity changes, it is faulty.
   • Circuit Voltage and Ground Integrity Test:
     • With the ignition ON and the hood switch connector disconnected, consult a wiring diagram to identify the signal wire and any ground or power supply wires leading to the switch.
     • Using the DMM, check for the specified reference voltage (typically 5V or battery voltage) on the signal wire from the BCM/SCM, relative to a known good chassis ground.
     • If a ground wire is present at the switch, check its continuity to chassis ground (should be very low resistance).
     • An incorrect or absent voltage indicates an open circuit or short back towards the BCM/SCM.
   • Wiring Harness Continuity Test:
     • With both the hood switch connector and the BCM/SCM connector disconnected, use the DMM in continuity mode.
     • Test for continuity between the signal wire terminal at the hood switch connector and its corresponding terminal at the BCM/SCM connector. There should be continuity (near 0 ohms).
     • Similarly, test the ground wire (if applicable) for continuity from the switch connector to its corresponding ground point or BCM/SCM terminal.
     • Also, check for any unintended continuity (shorts) between the signal wire and chassis ground or the signal wire and battery voltage.

Recommended Repairs and Solutions

Once the root cause of B1520 has been identified through diagnosis, the following repairs are typically recommended:

• Replace Hood Switch: If the DMM tests confirm the hood switch itself is faulty, replace it with a new OEM or high-quality aftermarket component. Ensure the new switch is properly installed and adjusted to actuate correctly when the hood is closed.
• Repair or Replace Damaged Wiring: For identified breaks, chafes, or severe corrosion in the wiring harness, perform a professional wire repair. This involves soldering the connection and using heat-shrink tubing for durability and environmental protection. If the damage is extensive, it may be more practical to replace the entire section of the harness. Ensure proper routing and securing of the wiring to prevent future damage.
• Clean and Secure Connectors: If corrosion or loose pins are found in the electrical connectors, clean the terminals using specialized electrical contact cleaner and a small brush. For bent or damaged pins, carefully straighten them or replace the terminal if necessary. Ensure all connectors are fully seated and securely latched.
• Adjust Hood Switch Mounting: If the hood switch is found to be functional but simply not making proper contact, adjust its mounting position or the hood latch striker mechanism to ensure full actuation when the hood is closed.
• BCM/SCM Replacement: Only consider replacing the BCM/SCM as a last resort, after all other components (switch, wiring, connectors) have been meticulously tested and confirmed to be in perfect working order. BCM/SCM replacement often requires specialized programming and synchronization with the vehicle’s immobilizer system, which typically necessitates dealership-level diagnostic tools and expertise.
• Clear DTC and Verify Repair: After any repair, use the OBD-II scan tool to clear the B1520 diagnostic trouble code. Cycle the hood several times, observing the live data parameter for the hood status to confirm that it now accurately reflects the hood’s physical state. Perform a complete drive cycle if necessary to ensure the code does not return.