What Does B1786 Mean?
This diagnostic trouble code indicates that the Body Control Module (BCM) has detected a malfunction in the electrical circuit controlling the downward movement of the passenger side exterior mirror. The BCM monitors current flow and circuit continuity in the mirror motor circuit and has identified either an open circuit, short circuit, or excessive resistance preventing proper downward mirror adjustment. This is a body system fault specific to the power mirror adjustment function.
Common Causes
40%
Faulty passenger side mirror motor assembly with failed downward movement motor winding or internal mechanical binding
30%
Damaged wiring harness or corroded connector between door and mirror assembly due to water intrusion or flexing fatigue
20%
Failed mirror control switch with internal contact degradation preventing proper ground or power delivery to down circuit
10%
Body Control Module internal relay or driver circuit failure for passenger mirror down function
Diagnostic Steps
1
Step 1: Visually inspect the passenger side mirror assembly for physical damage, water intrusion in the housing, and check for binding by manually moving the mirror glass through its full range of motion
2
Step 2: Using a digital multimeter, backprobe the passenger mirror connector at the mirror housing while activating the down function - verify presence of voltage (typically 12V) on the appropriate terminal and proper ground on the return circuit per manufacturer wiring diagram
3
Step 3: Disconnect the passenger mirror connector and perform continuity tests on the mirror motor down circuit wiring from the door harness connector back toward the BCM/switch module, checking for opens, shorts to ground, or shorts to power
4
Step 4: If wiring tests pass, connect a direct 12V power supply and ground to the mirror motor down circuit terminals (isolated from vehicle) to verify motor operation - motor should move mirror downward smoothly
5
Step 5: Test the mirror control switch by measuring resistance or continuity across switch terminals in the down position, or substitute with a known-good switch if available
6
Step 6: Clear codes, operate the mirror through all positions multiple times, and verify proper operation - if fault returns immediately with good motor and wiring, suspect BCM internal fault requiring module diagnosis or replacement
Estimated Repair Cost
$150 - $650
Parts + labor, varies by vehicle and location
This diagnostic trouble code indicates that the Body Control Module (BCM) has detected a malfunction in the electrical circuit controlling the downward movement of the passenger side exterior mirror. The BCM monitors current flow and circuit continuity in the mirror motor circuit and has identified ...
The most common cause of B1786 (Mirror Passenger Down Circuit Failure) is: Faulty passenger side mirror motor assembly with failed downward movement motor winding or internal mechanical binding
Typical repair costs for B1786 range from $150 to $650, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to drive with this fault as it only affects mirror adjustment convenience. This does not impact core vehicle safety systems, though proper mirror adjustment is important for visibility and should be addressed when convenient.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1786 to identify the root cause.
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Quick Info
Category
Body
System
Body Control and Accessory Systems
Difficulty
Type
Manufacturer
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