Passenger Mirror Down Circuit Short to Battery
Diagnostic trouble code B1788 indicates a short circuit to battery positive (B+) in the passenger side mirror downward movement control circuit. This code is specific to Toyota and Lexus vehicles and is triggered when the mirror control module (or body control module) detects an abnormally high voltage on the passenger mirror motor command circuit when it should be at a low or ground-switched state. The system monitors the resistance and voltage of the stepper or DC motor circuits that control the vertical tilt of the mirror.
1. Commonly Affected Makes
2. Most Common Causes
- Short circuit between the 'Mirror Passenger Down' control circuit wire and a B+ power wire
- Passenger mirror motor with internal winding shorted to positive
- Damaged, corroded, or water-infested mirror connector pins causing a short circuit
- Mirror control module (ECU) with a shorted output transistor
- Passenger door harness with insulation damaged due to friction or pinching
3. Symptoms You Will Notice
- Passenger side mirror does not move downward
- Mirror may move erratically or only in one direction
4. Step-by-Step Diagnosis
Visually inspect the passenger door harness and mirror connector for damage, corrosion, or water intrusion
Disconnect the mirror connector and measure resistance between the down control circuit and B+; a low resistance indicates a short
Test the mirror motor by applying 12V directly to the down terminal; if it moves, the motor is likely good
Check for a shorted output transistor in the mirror control module by measuring voltage at the down circuit with the module connected
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, the severity is low. The mirror may not adjust downward, but it does not affect vehicle safety or drivability.
How much does it cost to fix?▼
Repair costs typically range from $100 to $400, depending on whether the issue is a simple wiring repair or requires replacing the mirror assembly or control module.