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 8

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 8
  • 1Water ingress or terminal corrosion at the SRS ECU wiring harness connector. Driving through water, washing the vehicle, or a blocked sunroof drain tube causes water to flow under the center console (the typical SRS ECU location on Qin series models), shorting the CAN terminals to ground.
  • 2During front-end accident repairs or dashboard removal/installation, the metal frame and sheet metal pinch the wiring harness, damaging the insulation and causing the CAN line to contact the body ground point.
  • 3Chassis bottoming out or scraping split the floor wiring harness sleeve, causing the SRS communication harness to contact a sharp frame edge 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 wire.
  • 5Damaged CAN transceiver chip inside the SRS ECU (e.g., lightning strike, electrostatic breakdown, or power surge), causing the controller's internal CAN port to short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 3 minutes for the SRS system high-voltage capacitor to discharge fully, preventing 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 that will not 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). Check 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 harness-side CAN_H (refer to the specific model's 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 CAN_L (usually the orange/brown wire). This value must also be greater than 1 MΩ.
  • 5
    Voltage check: Reconnect the battery, turn the ignition switch to ON (do not start), and measure CAN_H to ground voltage (normal: 2.5-3.5 V) and CAN_L to ground voltage (normal: 1.5-2.5 V). A reading of 0 V 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 (usually through the floor channel, A-pillar, and instrument panel frame). Inspect the harness sleeve for wear, focusing on areas where it passes moving parts such as the door hinge, steering column, and pedal bracket.
  • 7
    Fault repair: After locating the damaged point, rewrap the wiring harness with high-temperature fleece tape (avoid using PVC tape). Adjust the wiring harness routing to maintain a clearance of at least 20mm from metal body parts. Repair or replace damaged connectors.
  • 8
    Terminal check: Measure the resistance between CAN_H and CAN_L (approximately 60Ω, the parallel value of two 120Ω terminating resistors). Confirm no internal short circuit in the ECU, then reconnect.
  • 9
    Clear and test: Clear the fault code. Perform a road test, including driving on bumpy roads. Confirm U1004-00 does not recur 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 8 Fault Codes
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.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.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.