AU
U1004-00

DTC U1004-00 indicates a short to ground in the CAN bus communication circuit of the BYD vehicle's airbag system (SRS) — Atto 3

Safety System

DTC U1004-00 indicates a short to ground in the CAN bus communication circuit of the BYD vehicle's airbag system (SRS).

In the BYD Qin series network architecture, the SRS ECU communicates in real time with the vehicle control unit, instrument cluster, and other modules via the CAN bus (typically powertrain CAN or body CAN).

If either CAN_H (high line, usually orange/black) or CAN_L (low line, usually orange/brown) shorts to body ground (GND) due to damaged insulation, water corrosion, or mechanical damage, the short pulls the differential bus signal voltage low and prevents normal transmission of communication messages.

This fault forces the SRS system into fail-safe mode, illuminates the airbag warning lamp on the instrument cluster, and prevents proper coordination between the seat belt pretensioners, crash sensors, and airbag deployment circuits.

This creates a severe safety hazard where the airbags fail to deploy during a collision.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Water ingress or terminal corrosion at the SRS ECU wiring harness connector. After vehicle wading, car washing, or a sunroof drain hose blockage, water flows under the center armrest (a common SRS ECU installation location on Qin series vehicles) and shorts the CAN terminals to ground.
  • 2Wiring harness pinched between the metal frame and sheet metal during front-end collision repairs or dashboard removal/installation, damaging the insulation and causing the CAN line to contact a body ground point.
  • 3Underbody bottoming out or scraping ruptured the floor wiring harness sleeve, causing the SRS communication wiring harness to contact a sharp edge on the chassis and short to ground.
  • 4During non-professional electrical modifications (such as installing a dash cam, ambient lighting, or audio system), fixing screws or cable ties pierce the SRS wiring harness, or tapping wires for power mistakenly grounds the CAN line.
  • 5Damaged internal CAN transceiver chip in the SRS ECU (e.g., from lightning strike, electrostatic discharge, or power surge), causing the controller's internal CAN port to short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes to completely discharge the SRS high-voltage capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use the BYD VDS or Launch X431 diagnostic tool to read the DTC. Confirm U1004-00 is a current fault and fails to clear. Record the freeze frame data.
  • 3
    Physical inspection: Locate the SRS ECU (usually under the center armrest box or near the gear selector mechanism on Qin 100/Qin EV models). Inspect the wiring harness connector for signs of water ingress, terminal corrosion, or backed-out pins.
  • 4
    Resistance measurement: Disconnect the SRS ECU connector. Use a multimeter to measure the resistance to ground at the harness-side CAN_H terminal (refer to the specific vehicle wiring diagram for the pin, usually the orange/black wire). The normal value must be greater than 1 MΩ. Measure the resistance to ground at the CAN_L terminal (usually the orange/brown wire). This value must also be greater than 1 MΩ.
  • 5
    Voltage verification: Reconnect the battery. Set the ignition switch to ON (do not start). Measure CAN_H to ground voltage (normal: 2.5-3.5V) and CAN_L to ground voltage (normal: 1.5-2.5V). A reading of 0V confirms a short to ground.
  • 6
    Wiring harness inspection: If resistance is abnormal, inspect the SRS wiring harness section by section along its routing path (typically through the floor channel, A-pillar, and instrument panel frame). Closely inspect the harness protective sleeve for wear at moving parts such as door hinges, the steering column, and pedal brackets.
  • 7
    Fault repair: After locating the damage, rewrap the wiring harness with high-temperature fleece tape (avoid PVC tape). Adjust the harness routing to maintain a clearance of at least 20mm from body metal parts. Repair or replace the damaged connector.
  • 8
    Terminal check: Measure the resistance between CAN_H and CAN_L (approximately 60 Ω, the parallel value of two 120 Ω terminating resistors). Confirm the ECU has no internal short circuit, then restore the connection.
  • 9
    Clear and test: Clear the fault code, perform a road test on a bumpy road, and confirm U1004-00 does not return and the airbag indicator turns off normally.
Real-World Cases
BYD DTC AI Analysis

Qin EV450 SRS system logged DTC U1004-00 after wading

✓
After driving through standing water, the instrument cluster displayed "Check SRS System" and the airbag warning light stayed on. Scanned with the VDS2000 diagnostic tool and retrieved current fault code U1004-00. Inspection found water pooling beneath the center console. The SRS ECU connector seal deteriorated, and internal terminal oxidation caused a CAN_H short to ground. Dried the wiring harness thoroughly, cleaned the connector terminals with electronic contact cleaner and applied conductive grease, replaced the waterproof seal, and rerouted the drain hose. No recurrence during the one-month follow-up after repair.
BYD DTC AI Analysis

Song MAX CAN network communication fault (similar to U1004-00 symptom)

✓
The vehicle occasionally cut out while driving, with multiple warning lights on the instrument panel. Diagnosis revealed loss of communication between the engine ECU and TCU. CAN bus voltage measurements showed anomalies characteristic of a short to ground. Investigation found a loose bolt on the left-side engine bay ground (G103), causing reference ground potential drift across the entire powertrain CAN network. Several modules logged CAN short-to-ground faults similar to U1004-00. Polished the ground point, applied conductive paste, and tightened the bolt to the specified torque of 10 N·m. Communication returned to normal.
Original source ↗
BYD DTC AI Analysis

Qin 100 U1004-00 DTC After Accident Repair

✓
Minor front collision. After the shop replaced the front bumper and dashboard, the SRS warning light stayed on. Read DTC U1004-00. Found the SRS main harness pinched between the dashboard crossmember and the AC duct bracket during the accident repair. A metal edge cut the insulation, shorting CAN_L to ground. Repair: Released the harness, repaired the wire insulation with heat-shrink tubing, rerouted the wiring per standard procedure, and installed a rubber grommet at the interference point. Fault resolved.
BYD DTC AI Analysis

Aftermarket modifications caused CAN short to ground on Qin 80

✓
After the owner installed an aftermarket 360-degree camera system, the SRS warning light came on. The scan tool showed code U1004-00 and could not clear it. Inspection found the installer had driven self-tapping screws through the firewall to secure the wiring harness, piercing the SRS harness sheath and shorting CAN_H and CAN_L to the body. Repair: Replaced the damaged SRS harness assembly (ECU-to-floor section), removed the self-tapping screws, secured the wiring with proper harness clips, and restored the factory routing.
BYD DTC AI Analysis

BYD Qin EV300 SRS ECU internal fault causing U1004-00

✓
No accident, water damage, or modification history. U1004-00 set suddenly during normal operation. Measured at the SRS ECU connector harness side: CAN_H and CAN_L both exceeded 10MΩ resistance to ground (normal), and harness voltage was normal. Connecting the ECU caused a short to ground at the connector. Diagnosed as a failed internal CAN transceiver chip in the SRS ECU. Replaced the airbag control module (required online coding and crash sensor calibration). Fault completely resolved.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.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.
Common problems reported for Atto 3 →Atto 3 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.