B1821: Rear Wiper Up Relay Circuit Short to Ground
The B1821 code indicates that the Body Control Module (BCM) has detected a short to ground in the control circuit for the rear wiper motor's 'Up' relay coil. This relay controls power to move the rear wiper arm to its parked or raised position. The fault prevents the rear wiper from operating correctly, typically causing it to stop working or function intermittently.
1. Commonly Affected Makes
2. Most Common Causes
- Internal short circuit in the rear wiper 'Up' relay coil
- Chafed or pinched wiring in the relay control circuit shorting to chassis ground
- Corroded, loose, or damaged terminals at the relay or BCM connector
- Internal failure of the BCM output driver
- Defective rear wiper relay (coil shorted)
3. Symptoms You Will Notice
- Rear wiper does not move or operates intermittently
- Rear wiper stuck in one position or fails to park
- Blown fuse for the rear wiper circuit
4. Step-by-Step Diagnosis
Verify the rear wiper fuse and replace if blown
Inspect the rear wiper relay for signs of damage or shorted coil (measure resistance across coil terminals)
Check wiring and connectors between the BCM and relay for shorts to ground or open circuits
Test the BCM output driver by commanding the relay on/off with a scan tool while monitoring voltage
If all else is normal, replace the BCM
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the rear wiper will not function, which may reduce visibility in rain or snow. The issue does not affect vehicle safety systems.
How much does it cost to fix?▼
Repair costs typically range from $100 to $400, depending on the cause. A relay replacement is usually under $100, while BCM replacement can exceed $400 including labor.