AU
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 U

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 U indicates a high-severity short to battery in the passenger-side mirror down adjustment circuit. The BCM may disable mirror movement to protect the circuit, and continued use can 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.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Aged and damaged front passenger-side 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).
  • 2The insulation layer of the internal coil in the rearview mirror adjustment motor burned out, causing a short circuit between the positive and negative terminals and abnormally low resistance (normally 15-25Ω, close to 0Ω when short-circuited).
  • 3After replacing the door mirror assembly with an aftermarket unit, mismatched internal circuit pinouts or resistance values cause the BCM to incorrectly 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 allows water to enter and corrode 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 to ensure the diagnostic system resets.
  • 2
    Remove the front passenger door trim panel. Inspect the wiring harness sleeve at the door-to-body connection. Pull back the corrugated conduit to inspect the wire insulation for wear. Repair and re-wrap any damaged wiring.
  • 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, a short circuit exists. Measure the motor resistance. If the resistance falls outside the standard range (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. A 12V output without operation indicates an internal BCM fault.
  • 5
    After repairing the short circuit source, clear the fault code and perform rearview mirror position initialization learning (some models require using VDS to perform the "Exterior Rearview Mirror Memory Learning" function). Verify that adjustment in all directions 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 U Fault Codes
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.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.