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) — Seal U

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 — Seal U
  • 1Water ingress or terminal corrosion at the SRS ECU wiring harness connector. This occurs especially after driving through water, washing the vehicle, or a sunroof drain hose blockage. Water flows under the center armrest box (a common SRS ECU installation location on Qin series models) and shorts the CAN terminals to ground.
  • 2Wiring harness pinched between the metal frame and sheet metal during front-end accident repairs or dashboard removal and installation, 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 wiring harness to contact a sharp edge on the frame and short to ground.
  • 4During non-professional electrical modifications (such as installing dash cams, ambient lighting, or audio systems), fixing screws or cable ties pierce the SRS wiring harness, or tapping into wires for power mistakenly grounds the CAN wire.
  • 5Damaged 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 battery negative terminal and wait at least 3 minutes for the SRS system high-voltage capacitors to discharge fully to avoid the risk of 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 cannot be cleared. 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 vehicle wiring diagram for the pin; usually the orange/black wire). The normal value should be greater than 1 MΩ. Measure the resistance to ground at CAN_L (usually the orange/brown wire). This value should also be greater than 1 MΩ.
  • 5
    Voltage check: Reconnect the battery, turn the ignition switch to ON (do not start), and measure the CAN_H to ground voltage (normal: 2.5-3.5V) and CAN_L to ground voltage (normal: 1.5-2.5V). A voltage of 0V confirms a short to ground.
  • 6
    Harness inspection: If resistance is abnormal, inspect the SRS harness section by section along its routing (usually through the floor channel, A-pillar, and instrument panel frame). Check the harness sleeve for wear where it passes through moving parts such as door hinges, the steering column, and the pedal bracket.
  • 7
    Fault repair: Upon locating the damage, rewrap the wiring harness using high-temperature fleece tape (avoid PVC tape). Adjust the harness routing to maintain at least 20mm clearance from metal body parts. Repair or replace the damaged connector.
  • 8
    Terminal verification: Measure the resistance between CAN_H and CAN_L (approximately 60Ω, the parallel value of two 120Ω terminating resistors). Confirm no internal short circuit exists in the ECU, then reconnect.
  • 9
    Clear and test: Clear the fault code, perform a road test on a bumpy road, and verify 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 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.