What Does B1821 Mean?
This diagnostic trouble code indicates that the Body Control Module (BCM) has detected a short to ground in the control circuit for the rear wiper motor up relay coil. The BCM monitors the relay coil circuit and sets this code when it detects abnormally low resistance or voltage, indicating the circuit is shorted to chassis ground. This prevents proper operation of the rear wiper motor in the upward direction.
Common Causes
45%
Damaged or chafed wiring harness between BCM and rear wiper relay causing short to ground
30%
Failed rear wiper motor up relay with internal short to ground
15%
Corroded or water-damaged connector terminals at relay or BCM causing ground path
10%
Faulty Body Control Module with internal short in relay driver circuit
Diagnostic Steps
1
Step 1: Perform visual inspection of rear wiper relay, wiring harness from BCM to relay location, and all connectors for visible damage, corrosion, water intrusion, or chafed insulation against metal surfaces
2
Step 2: Disconnect rear wiper motor up relay from its socket and use a digital multimeter to measure resistance between the relay coil control wire terminal (at harness side) and known good chassis ground. Reading should be infinite ohms (open circuit); low resistance indicates short to ground in wiring
3
Step 3: If wiring tests good, test the relay itself by measuring resistance across relay coil terminals with multimeter. Should show specified coil resistance (typically 60-120 ohms). Also check for continuity between either coil terminal and relay case (should be infinite ohms)
4
Step 4: If relay and wiring test good, inspect BCM connector for corrosion, pushed-back pins, or moisture. Disconnect BCM and measure resistance between the relay control circuit pin at BCM harness connector and ground. Should be infinite ohms
5
Step 5: Repair or replace identified faulty component (wiring, connector, relay, or BCM). Clear codes and cycle rear wiper through all modes to verify proper operation and confirm no code returns
Estimated Repair Cost
$100 - $450
Parts + labor, varies by vehicle and location
This diagnostic trouble code indicates that the Body Control Module (BCM) has detected a short to ground in the control circuit for the rear wiper motor up relay coil. The BCM monitors the relay coil circuit and sets this code when it detects abnormally low resistance or voltage, indicating the circ...
The most common cause of B1821 (Wiper Rear Motor Up Relay Coil Circuit Short To Ground) is: Damaged or chafed wiring harness between BCM and rear wiper relay causing short to ground
Typical repair costs for B1821 range from $100 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This code does not present a safety risk and the vehicle is safe to drive. Rear wiper functionality may be impaired which could reduce rear visibility in inclement weather, so repair is recommended before driving in rain or snow.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1821 to identify the root cause.
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Quick Info
Category
Body
System
Body Control System - Rear Wiper
Difficulty
Type
Manufacturer
Recommended Tools
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