What Does B1780 Mean?
This diagnostic trouble code indicates that the Body Control Module (BCM) has detected a short to battery voltage in the driver's side mirror down motor circuit. The BCM commands the mirror glass to tilt downward, but instead of seeing normal motor operation or ground reference, it detects constant battery voltage on the circuit that should be switched. This suggests unwanted voltage is present where it shouldn't be, preventing proper mirror adjustment functionality.
Common Causes
40%
Damaged or chafed wiring in the door jamb area causing the down motor circuit to contact a power wire
30%
Failed mirror switch assembly with internal short circuit feeding battery voltage to the down circuit
20%
Faulty driver side mirror motor assembly with internal short to power
10%
Corrosion or water intrusion in mirror connector causing cross-circuit contamination with power circuit
Diagnostic Steps
1
Step 1: Perform visual inspection of the driver's door mirror wiring harness, especially in the door jamb flex area where wires repeatedly bend. Look for chafed insulation, exposed copper, or damage from door closure. Check mirror and door connectors for corrosion, pushed-out pins, or moisture intrusion.
2
Step 2: Disconnect the driver's side mirror connector at the mirror assembly. Using a digital multimeter, backprobe the down motor circuit wire at the door harness side (vehicle side) with key on. You should see 0 volts with no switch input. If battery voltage is present, the short is between the BCM and the mirror connector.
3
Step 3: If voltage was found in Step 2, disconnect the mirror switch assembly connector. Recheck voltage at the mirror harness connector. If voltage disappears, the switch is shorted internally. If voltage remains, the short is in the door or body wiring harness between switch and mirror.
4
Step 4: If no voltage was found at the harness in Step 2, reconnect the mirror and measure resistance between the down motor circuit pins at the mirror connector with mirror disconnected. Normal reading should be 3-8 ohms for the motor. Reading of less than 1 ohm to the B+ pin indicates internal mirror motor short.
5
Step 5: After isolating and repairing the short circuit (replacing switch, mirror, or repairing wiring), clear the code and cycle the mirror through all positions using the switch. Verify smooth operation in all directions and confirm no codes return.
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 battery voltage in the driver's side mirror down motor circuit. The BCM commands the mirror glass to tilt downward, but instead of seeing normal motor operation or ground reference, it detects constant ...
The most common cause of B1780 (Mirror Driver Down Circuit Short To Battery) is: Damaged or chafed wiring in the door jamb area causing the down motor circuit to contact a power wire
Typical repair costs for B1780 range from $100 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This is safe to continue driving as it only affects power mirror adjustment functionality and does not impact vehicle operation or safety systems. Repair at your convenience, though the mirror adjustment may be inoperative or stuck in one position.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1780 to identify the root cause.
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Quick Info
Category
Body
System
Power Mirror System
Difficulty
Type
Manufacturer
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