What Does B1778 Mean?
DTC B1778 indicates a malfunction in the driver's side power mirror down movement circuit. The Body Control Module (BCM) or Door Module has detected an abnormal voltage, current draw, or open/short circuit condition when commanding the mirror to tilt downward. This code specifically affects the vertical downward adjustment motor circuit of the driver's side exterior mirror.
Common Causes
40%
Failed mirror motor or internal motor winding short/open circuit in the downward direction
30%
Damaged or corroded wiring harness between door module and mirror assembly, including broken wires or water intrusion in door jamb
20%
Faulty driver door module or mirror switch assembly with internal contact failure
10%
Poor ground connection at mirror assembly or door module causing voltage feedback
Diagnostic Steps
1
Step 1: Perform visual inspection of driver side mirror housing for physical damage, water intrusion, or loose mounting. Check mirror control switch for proper physical operation and listen for motor noise when commanding down movement.
2
Step 2: Using a digital multimeter, backprobe the mirror connector at the mirror assembly with ignition on and measure voltage on the down circuit wire while activating the switch. Verify presence of battery voltage (typically 12-14V) during activation, then check for proper ground on return circuit.
3
Step 3: If voltage is present at mirror connector, disconnect mirror and measure resistance across the down motor terminals (typically 3-8 ohms). Compare to up motor circuit resistance. Excessive resistance (>20 ohms) or infinite resistance indicates motor failure.
4
Step 4: If no voltage reaches mirror connector, inspect door jamb boot area for pinched or broken wires using multimeter continuity test from door module connector to mirror connector on both power and ground circuits.
5
Step 5: Check door module connector for corrosion, backed-out pins, or moisture. Measure voltage output from door module connector on down circuit command wire while operating switch to isolate module vs wiring fault.
6
Step 6: Clear codes, perform mirror relearn/calibration procedure if available via scan tool, then cycle mirror through full range of motion multiple times and verify code does not return.
Estimated Repair Cost
$150 - $600
Parts + labor, varies by vehicle and location
DTC B1778 indicates a malfunction in the driver's side power mirror down movement circuit. The Body Control Module (BCM) or Door Module has detected an abnormal voltage, current draw, or open/short circuit condition when commanding the mirror to tilt downward. This code specifically affects the vert...
The most common cause of B1778 (Mirror Driver Down Circuit Failure) is: Failed mirror motor or internal motor winding short/open circuit in the downward direction
Typical repair costs for B1778 range from $150 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving as this only affects mirror adjustment convenience feature. Mirror can typically still be adjusted manually or may function in other directions, though reduced visibility due to improper mirror positioning could present safety concerns in certain driving conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1778 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Body
System
Body Control Systems - Power Mirror
Difficulty
Type
Manufacturer
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
