OBD2 Code B1347: Rear Defogger Relay Circuit Fault
The Body Control Module (BCM) has detected a voltage mismatch in the rear defogger relay control circuit. This means the actual voltage on the control wire does not match the commanded state (open or closed) from the BCM, indicating an open circuit, short to ground, or short to battery in the wiring or relay.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty rear defogger relay
- Open or shorted wiring between BCM and relay
- Blown fuse in the battery junction box (BJB)
- Damaged, loose, or corroded relay or BCM connector
- Internal BCM failure
3. Symptoms You Will Notice
- Rear defogger inoperative
- Rear defogger indicator light on dash may not illuminate or stay on
4. Step-by-Step Diagnosis
Check and replace any blown fuses in the battery junction box related to the rear defogger.
Inspect the rear defogger relay and its connector for damage, corrosion, or loose pins.
Test the relay by applying battery voltage to the coil terminals and checking for continuity across the switch terminals.
Measure voltage at the BCM output for the rear defogger relay control circuit with a multimeter while commanding the defogger on/off.
If wiring and relay are good, suspect a faulty BCM and perform BCM self-test or replacement.
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the rear defogger will not work, which can reduce visibility in cold or humid conditions.
How much does it cost to fix?▼
Repair costs typically range from $50 to $200 for relay or fuse replacement, up to $500-$1000 if BCM replacement is needed.