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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
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

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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
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.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.
Data confidence: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.