B1521

Hood Switch Circuit Short To Battery

Body Chassis/Safety Hood Switch Fault 🟢 Low — Fix at next service ✅ Safe to Drive
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What This Actually Means

In plain language — no jargon

The hood switch circuit is shorted directly to battery voltage, meaning the electrical connection is bypassed and sending constant high voltage to the ECU. It's like a light switch stuck in the 'on' position, continuously sending the wrong signal.

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Symptoms You May Notice

3 known symptoms for this code
Hood ajar warning light stays on or malfunctions
ECU cannot detect when hood is actually open or closed
Possible electrical drain or module communication issues
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the hood switch signal voltage, expecting it to toggle between ground (closed) and a reference voltage (open). When shorted to battery, the signal remains at battery voltage continuously, preventing the ECU from detecting state changes. The module compares this voltage against normal operational thresholds to identify the fault condition.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
Hood Switch Voltage 0V (closed) to 5V (open) Constant battery voltage (12-14V)
Signal State Changes Transitions when hood opens/closes No voltage variation detected
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Hood switch connector
Inspect and reseat the hood switch connector at the latch to ensure proper contact and eliminate loose pin faults.
2
Hood switch wiring harness
Check for damaged or pinched wires along the hood harness routing that may be causing a short to nearby battery or power lines.
3
Hood switch assembly
Replace the hood switch if internal contacts are welded closed or the switch cannot be reset by disconnecting battery power for 30 minutes.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code B1521 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code B1521

What happens after you fix the fault

Once the fault is repaired, B1521 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.