AU
U1004

In BYD new energy vehicles, DTC U1004 typically indicates "loss of communication with the Vehicle Control Unit (VCU)" or "powertrain CAN bus off" — Atto 3

Safety System

In BYD new energy vehicles, DTC U1004 typically indicates "loss of communication with the Vehicle Control Unit (VCU)" or "powertrain CAN bus off".

Some SRS system documentation defines it as "CAN short to ground".

This fault indicates a physical layer abnormality in the Controller Area Network (CAN) communication bus.

Specifically, the insulation resistance to ground for CAN-H (high line) or CAN-L (low line) decreases or shorts completely.

This pulls the bus differential signal voltage down to ground potential, preventing normal data frame transmission.

Consequently, this interrupts communication between the VCU or airbag control unit and other key vehicle modules, such as the Battery Management System (BMS), Motor Control Unit (MCU), and Instrument Cluster Unit (ICU).

The interruption triggers the fault protection mechanism, forcing the vehicle into limp mode (speed limited to 20-40 km/h) or inhibiting the Ready state, severely compromising driving safety.

In the SRS system, this fault also disables passive safety systems such as the airbags and seat belt pretensioners.

The U1004 fault code on the Atto 3 indicates a high-severity CAN communication failure, often a loss of communication with the VCU or a CAN line short to ground. Common causes include a blown VCU power fuse, damaged wiring, corroded connectors, or water ingress at control module plugs. The vehicle may enter limp mode or fail to become Ready. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Abnormal VCU or SRS control unit power supply circuit: blown VCU power supply fuse in the front compartment fuse box (usually F1/23 or F2/08, 15A), burnt relay contacts, or wiring harness wear at the firewall or floor causing a short to ground.
  • 2CAN bus physical layer damage: Power CAN or Comfort CAN wiring harness chafing against air conditioning lines or sharp body edges; aging waterproof seals causing water ingress and corrosion after wading (common at gateway module GWC or VCU connectors); backed-out connector pins, oxidation, or excessive contact resistance.
  • 3Internal control module fault: internal CAN transceiver chip breakdown in the VCU, Gateway Controller (GWC), or Airbag Control Unit; power management circuit fault causing abnormal 12V power output to the CAN line; or module failing to enter sleep mode, continuously occupying the bus.
  • 412V low-voltage system fault: Aging or discharged battery (voltage <10V) or excessive quiescent current (parasitic draw >50mA) causes insufficient VCU power supply or repeated restarts, triggering a communication timeout.
  • 5Software or modification factors: The VCU/GWC software version contains a CAN bus load management defect, or unauthorized parallel connection of aftermarket accessories (such as 360-degree surround-view cameras or dash cams) to the powertrain CAN bus causes excessive bus load or signal reflection.
What To Do
  • 1
    Use VDS or a dedicated diagnostic tool to read the complete fault code stream. Confirm whether U1004 is an active or history fault. Record accompanying fault codes (e.g., U0100, U0110, B1C00) and preliminarily determine if the issue is a VCU communication fault or an SRS system fault.
  • 2
    Check the power and ground of the VCU (usually located under the front passenger floor) or the SRS control unit. Measure the constant power (B+) and ignition power (IG); the voltage should be 11-14V. Measure the ground point resistance; it should be less than 0.5Ω. Check the relevant fuse in the front compartment power distribution box. If blown, measure the downstream circuit resistance to ground to locate the short circuit.
  • 3
    Measure the CAN bus physical layer at the OBD diagnostic port: check pins 6 (CAN-H) and 14 (CAN-L). Terminating resistance should be approximately 60 Ω (two 120 Ω resistors in parallel). Normal CAN-H to ground voltage is 2.5-3.5 V, and normal CAN-L to ground voltage is 1.5-2.5 V. Voltage near 0 V with infinite resistance indicates a short to ground or an open circuit.
  • 4
    Check the wiring harness in sections: disconnect the VCU/SRS connectors and measure the insulation resistance to ground of the CAN wires on the harness side. A resistance below 10 kΩ indicates a short circuit. Follow the CAN wire routing and inspect the firewall, center console, and front compartment harness fixing points. Focus on signs of water ingress and chafing against air conditioning pipes and metal brackets.
  • 5
    Check the gateway module (GWC): If multiple systems report communication faults simultaneously, remove the center console to inspect the GWC connector (usually located on the right side of the air conditioning evaporator housing) for corrosion or backed-out pins. Check if the air conditioning drain hose has detached, causing water to enter the connector.
  • 6
    Module-level diagnosis: If the wiring harness is normal, check the VCU/SRS connectors for backed-out pins or oxidation; attempt a VCU software update (if a newer version is available); if necessary, substitute the module to verify an internal module fault.
  • 7
    Verify repair: Clear all fault codes, perform a road test under multiple conditions (including bumpy roads and high-speed driving), use the diagnostic tool to monitor the data stream for normal operation, and confirm the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Pro DM (2019) - VCU Power Supply Fault Caused Communication Loss

✓
Symptoms: Vehicle failed to start after sitting overnight. The dashboard displayed "Power System Fault" and the Ready light did not illuminate. Diagnosis: Scan tool retrieved U1004 (Communication fault with Vehicle Control Unit) and B1C00 (VCU system fault). CAN-H voltage at the diagnostic port measured 0.2 V (normal 2.5–3.5 V) and CAN-L measured 4.8 V (normal 1.5–2.5 V), indicating CAN bus failure. Disconnected the VCU connector (located under the passenger floor) and found no 12 V supply at pins 1 and 2 on the VCU side. Inspected fuse F1/23 (15 A, VCU constant power) in the engine compartment fuse box; it was blown. Resistance to ground showed an intermittent short. Resolution: Replaced the VCU power supply fuse. Traced the harness along the firewall and found it rubbing against the air-conditioning pipe, causing the short. Re-wrapped the harness and adjusted the routing to avoid interference. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD e5 (2018 model, ride-hailing vehicle) – Gateway module water ingress corrosion

✓
Symptoms: After driving in wet conditions, the vehicle suddenly lost power. The instrument cluster displayed "Check Powertrain System" and speed was limited to 20 km/h. Retrieved fault codes U1004 (VCU Communication Timeout) and U0100 (Lost Communication with Motor Controller). Diagnosis: Checked VCU power and earth - normal. Measured the powertrain CAN bus and found infinite resistance between CAN-H and CAN-L (normal value 60 Ω), with abnormal voltage to earth. Measuring sections of the CAN wiring revealed severe corrosion on pins 10 and 11 (powertrain CAN) of connector B at the gateway controller (located inside the centre console, right side of the A/C evaporator). Disassembled the gateway module and found water ingress on the internal circuit board that prevented CAN signal forwarding, paralyzing the entire powertrain CAN network. Solution: Replaced the gateway control module. Fixed the A/C drain hose leak (the drain hose had detached, allowing water to enter the centre console). Resealed the wiring harness connectors. Test drove for one week with no recurrence.
Original source ↗
BYD DTC AI Analysis

BYD Song DM (2017) - VCU Software Version Defect

✓
Symptoms: Intermittent juddering while driving, followed by "Check Engine" and "Check Powertrain" warnings on the dashboard and limp mode. Normal operation returned after stopping and restarting. Fault occurred approximately 1-2 times per week. Historical fault code U1004 (VCU communication loss). Diagnosis: Checked VCU wiring harness and connectors; no looseness. Power and ground circuits normal. CAN bus waveform testing showed no abnormalities. Comparison with an identical model revealed the VCU software was the early 2017 version, which has a known defect of occasional communication interruption under high CAN bus load. Found an aftermarket 360-degree camera system (non-OEM) installed, consuming additional CAN bus bandwidth and causing communication conflicts. Solution: Upgraded VCU software to the March 2019 version (VCU_20190315). Removed the aftermarket 360-degree camera system and restored factory wiring. Cleared fault codes. No recurrence after one month of monitoring.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM (2020) — CAN Wiring Harness Poor Contact

✓
Symptoms: Sudden power loss during high-speed driving. The instrument cluster blacked out for 2 seconds, then recovered and displayed "Powertrain Malfunction". Pure electric mode was unavailable, but the vehicle could drive at low speeds in hybrid mode. The scan tool showed U1004 (VCU Communication Fault) as a current fault that would not clear. Diagnosis: Inspected the VCU wiring harness connector in the front compartment. Found three pins inside the 64-pin connector (CAN-H, CAN-L, and power) had backed out with insufficient insertion depth. Disassembling the connector revealed the internal locking tab had broken, allowing the connector to loosen from driving vibrations and causing intermittent power and communication loss to the VCU. The backed-out power pin measured 15Ω contact resistance (normal <0.5Ω), resulting in insufficient VCU operating voltage. Fix: Replaced the VCU vehicle wiring harness connector (repair kit). Re-crimped the pins to ensure full insertion and added a cable tie at the connector to prevent loosening during driving. Road-tested 100 km with no fault recurrence.
Original source ↗
BYD DTC AI Analysis

BYD e2 (2021 model) - Low 12V battery voltage causing communication fault

✓
Symptoms: No start after 5 days parked. Dashboard showed "Check Power System". Scanned DTCs U1004 (VCU communication fault) and U0110 (lost communication with battery manager). 12V battery voltage: 9.8V. Diagnosis: Jump-started with external power. Codes cleared; vehicle drove normally. Confirmed as secondary fault from discharged 12V battery. Checked parasitic draw: 2.8A after locking (normal <50mA). Found VCU failed to enter sleep mode after shutdown, continuously transmitting CAN signals. Cause: internal VCU wake-up circuit fault. Repair: Replaced 12V battery (original damaged) and VCU control unit. Parasitic draw dropped to 35mA. Fault resolved. Advised owner to disconnect negative battery terminal for extended parking.
Original source ↗
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.