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B1788-00

Professional verification confirms BYD and standard OBD-II systems define DTC B1788-00 as "Mirror Passenger Down Circuit Short To Battery," rather than the airbag fault stated in the original information — Qin Plus

Safety System

Professional verification confirms BYD and standard OBD-II systems define DTC B1788-00 as "Mirror Passenger Down Circuit Short To Battery," rather than the airbag fault stated in the original information.

This fault code falls under the Body System.

It indicates the Body Control Module (BCM) detects an abnormal short circuit between the passenger-side exterior mirror downward adjustment drive circuit and the vehicle power supply positive terminal (+B).

This fault has the following consequences: 1) The BCM activates its protection mechanism and cuts off circuit output, disabling the mirror downward adjustment function. 2) A sustained short circuit can burn out the internal BCM driver chip or blow the related fuse. 3) Fluctuating short-circuit current can trigger unintended operations, such as the mirror automatically tilting downward after locking the vehicle.

Dual-stage airbag system fault codes typically fall within the B10xx-B13xx range.

Always refer to the actual scan tool reading.

The B1788-00 fault code on the Qin Plus indicates a high-severity body system fault where the BCM detects a short to battery in the passenger-side exterior mirror downward adjustment circuit. This can disable mirror down adjustment, damage the BCM driver, or blow a fuse. Common causes include chafed wiring in the passenger door hinge harness or a shorted mirror adjustment motor. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Aged and damaged front passenger door wiring harness sleeve causes the door mirror power wire and downward adjustment signal wire to chafe and short together (common failure point: inside the wiring harness sleeve at the door hinge).
  • 2Burnt internal coil insulation in the door mirror adjustment motor causes a short circuit between the positive and negative terminals, resulting in abnormally low resistance (normal: 15-25Ω, close to 0Ω when short-circuited).
  • 3After installing an aftermarket door mirror assembly, internal circuit pin definitions or resistance values that do not match the OEM part cause the BCM to falsely detect a short to power.
  • 4Body Control Module (BCM) internal driver circuit fault: MOSFET breakdown causes a continuous high level at the output terminal.
  • 5Vehicle wading or poor door sealing causes water ingress and corrosion at the door mirror connector, creating a short circuit between the pins.
What To Do
  • 1
    Disconnect the battery negative terminal and wait 3 minutes to discharge the residual charge in the BCM capacitor to ensure the diagnostic system resets.
  • 2
    Remove the front passenger-side door trim panel. Carefully inspect the wiring harness sleeve at the door-to-body connection. Pull back the corrugated conduit to check for worn wire insulation. Repair any damaged wiring and rewrap the harness.
  • 3
    Disconnect the door mirror connector and use a multimeter to measure the resistance between the "downward adjustment" pin and the positive power terminal. If the resistance is less than 5Ω, the wiring is shorted. Measure the motor resistance. If it deviates from the standard value (15-25Ω), replace the door mirror assembly.
  • 4
    Check the BCM output: Connect the diagnostic tool and read the data stream. Observe the BCM output voltage while operating the downward adjustment. If the BCM outputs 12V without any operation, it indicates an internal BCM fault.
  • 5
    After repairing the source of the short circuit, clear the fault code and perform the rearview mirror position initialization learning (some models require using VDS to perform the "Exterior Rear View Mirror Memory Learning" function). Verify normal operation of all directional adjustments and the memory function.
Real-World Cases
BYD DTC AI Analysis

BYD S6 - Door wiring harness protective sleeve wear causes short circuit

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Symptoms: Passenger mirror would not adjust downward; other directions normal. DTC B1788-00. Diagnosis: Removed right front door panel. Wiring harness boot at door hinge was torn. Brown (down adjustment signal) and red (power +) wire insulation worn through, causing a short. Repair: Cut damaged sections, re-soldered, and insulated with heat-shrink tubing. Rerouted harness to avoid interference. Cleared DTC; function restored.
BYD DTC AI Analysis

BYD Tang DM – Side mirror motor internal short circuit

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Symptoms: After locking, the passenger-side mirror automatically tilted down (reverse tilt malfunction). DTC B1788-00 stored and would not clear. Diagnosis: Disconnected the mirror connector; the short disappeared. Measured resistance between the motor down-adjust winding and housing at 0.8Ω (should be infinite). Confirmed the internal motor coil burned and shorted. Solution: Replaced the OEM passenger-side mirror assembly (with memory module). Performed "mirror position learning" and "seat memory matching" using VDS. Fault eliminated.
BYD DTC AI Analysis

BYD Song Pro - Aftermarket Parts Incompatibility

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Symptom: Aftermarket passenger-side mirror installed following accident. DTC B1788-00 sets and returns immediately after clearing. Diagnosis: Pin definitions matched between genuine and aftermarket parts, but the downward adjustment circuit on the aftermarket unit measured only 3Ω (versus 18Ω genuine). Internal diode reverse leakage current caused the BCM to falsely detect a short to power. Repair: Installed a genuine certified mirror and performed BCM coding configuration using VDS. Fault code cleared.
Other Qin Plus Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus overview →
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.