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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 — Seal 6 EV

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 Seal 6 EV indicates a high-severity body system fault where the BCM detects a short to battery in the passenger-side mirror downward adjustment circuit. This can disable mirror down movement and may damage the BCM driver or fuse. Common causes include chafed door-hinge wiring or a shorted mirror adjustment motor. Visit an authorised BYD dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Aged and damaged front passenger door wiring harness sleeve causes the door mirror power wire and downward adjustment signal wire to chafe and short circuit (common location: inside the wiring harness sleeve at the door hinge).
  • 2Burned insulation on the rearview mirror adjustment motor internal coil causes a short circuit between the positive and negative terminals, resulting in abnormally low resistance (normal: 15-25Ω, short circuit: close to 0Ω).
  • 3After replacing the door mirror assembly with an aftermarket unit, the internal circuit pinout or resistance values do not match the OEM part, causing the BCM to falsely detect a short to power.
  • 4Body Control Module (BCM) internal drive circuit fault; MOSFET breakdown causes a continuous high level at the output terminal.
  • 5Vehicle wading or poor door sealing causes water ingress and corrosion in the rearview 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 and reset the diagnostic system.
  • 2
    Remove the front passenger 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 re-wrap the harness.
  • 3
    Disconnect the door mirror connector. Use a multimeter to measure the resistance between the "down adjustment" pin and the positive power supply terminal. If the resistance is less than 5Ω, the wiring has a short circuit. Measure the motor resistance. If it falls outside the standard range (15-25Ω), replace the door mirror assembly.
  • 4
    Check 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 operation, it has an internal fault.
  • 5
    After repairing the short circuit, clear the fault code and perform rearview mirror position initialization learning (on some models, use VDS to perform the "Exterior Rear View Mirror Memory Learning" function). Verify that all directional adjustments and the memory function operate normally.
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 Seal 6 EV 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.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.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 Seal 6 EV →Seal 6 EV 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.