AU
U011887

DTC U011887 is a U-category CAN bus communication fault indicating a subnet communication interruption between the AC Controller and the Battery Cooling Controller (BCC) — Atto 8

Thermal Management System

DTC U011887 is a U-category CAN bus communication fault indicating a subnet communication interruption between the AC Controller and the Battery Cooling Controller (BCC).

In the BYD Qin EV300 architecture, the air conditioning system uses an independent CAN subnet.

The BCC manages the battery pack liquid cooling system, controlling components such as the battery cooling water pump and the battery cooler three-way valve.

The AC Controller requires real-time cooling demand and status information from the BCC to coordinate refrigerant distribution between the cabin air conditioning and the battery cooling system.

The AC Controller triggers this DTC when it fails to receive valid CAN messages from the BCC for a continuous period (typically multiple message cycles of 100–200 ms).

These messages include IDs containing signals for battery cooling requests, temperature, and flow rate.

This fault forces the thermal management system into a degraded mode.

This limits battery cooling capacity, which may reduce fast charging speeds or cause battery overheating.

The air conditioning system may also forcibly restrict cooling output or enter limp-home protection mode.

However, this fault typically does not cause a complete vehicle breakdown.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1A/C sub-network CAN wiring harness fault: CAN-H or CAN-L circuit open, short to ground, short to power, or poor connection due to the high-temperature, high-humidity environment in the front compartment, especially wiring harness corrosion caused by wading or mud and water ingress near the right front wheel arch (common BCC mounting location).
  • 2BCC power supply or ground fault: blown BCC constant power (+B) fuse, poor relay contact, or loose or oxidized ground point (G point), causing the controller to intermittently lose power or reset, preventing CAN communication.
  • 3Poor connector contact: An aged sealing ring on the BCC 24-pin (or 32-pin) connector causes terminal back-out, oxidation, and water ingress corrosion on the pins (especially CAN communication pins), interrupting signal transmission.
  • 4BCC internal fault: Damaged internal CAN transceiver chip (TJA1042 or similar), failed power management chip, or software crash preventing message transmission.
  • 5Abnormal terminating resistance: The air conditioning sub-network has a 120Ω terminating resistor at the BCC or AC end. An open circuit or resistance drift (due to water ingress) causes CAN signal reflection, degrading communication quality until communication fails.
What To Do
  • 1
    Freeze frame analysis: Use VDS2000/3000 or a BYD dedicated diagnostic tool to read the fault freeze frame. Confirm the vehicle speed, ambient temperature, and air conditioning status at the time of the fault to determine if it is an intermittent fault (e.g., on rough roads or after driving through water).
  • 2
    Basic power supply check: Disconnect the BCC connector and measure the voltage at the BCC power supply pins (usually Pins 1/2 are +B, Pins 3/4 are ground). The voltage should be 12V±0.5V, and the ground resistance should be less than 1Ω. Inspect the BCC-related fuse in the front compartment power distribution box (e.g., F1/16 15A) for a blown element or poor contact.
  • 3
    CAN bus physical layer check: Measure the terminating resistance between CAN-H (usually Pin 11 or Pin 21, yellow wire) and CAN-L (usually Pin 22 or Pin 12, green wire) at the BCC connector. Turn off the ignition switch. The resistance should be approximately 60 Ω (two 120 Ω resistors in parallel). A 120 Ω reading indicates one missing terminating resistor; an infinite reading indicates an open circuit; 0 Ω indicates a short circuit.
  • 4
    CAN line voltage and waveform check: Turn the ignition switch ON. Measure the CAN-H to ground voltage (2.5-2.7V) and the CAN-L to ground voltage (2.3-2.5V); the difference between the two is approximately 0.2V. Use an oscilloscope to verify a standard CAN waveform without distortion or noise interference.
  • 5
    Harness continuity and insulation test: Measure CAN line continuity from the air conditioning controller to the BCC; resistance must be less than 1Ω. Measure CAN line insulation resistance to the vehicle body; resistance must be greater than 10MΩ. Inspect the harness sleeve for damage, focusing on the front longitudinal beam and firewall pass-through.
  • 6
    Substitution verification: If the wiring harness is normal, first update the BCC software to the latest version. If the fault persists, cross-check the BCC unit by substitution (swap with a unit from the same vehicle model or install a new part). Finally, verify the air conditioning controller.
  • 7
    Fault Clearing and Road Test Verification: After repair, clear the fault code and perform a road test of at least 20 km, including bumpy roads, maximum air conditioning cooling mode, and DC fast charging conditions, to verify the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Worn wiring harness in right front wheel arch caused intermittent communication interruption

✓
A 2017 Qin EV300. The customer reported the air conditioning suddenly blew hot air when driving on rough roads, and the coolant temperature warning light on the instrument panel lit intermittently. The scan tool retrieved U011887 (current fault) and U011187 (history fault). Inspection found the BCC mounted behind the right front wheel arch liner; its harness retaining clip had detached, allowing the harness to rub against the wheel arch metal edge when the vehicle hit bumps. This wore through the CAN-L wire insulation and caused an intermittent open circuit. Soldered and heat-shrunk the harness, re-secured the routing with cable ties, and added a rubber protective sleeve. Fault resolved.
BYD DTC AI Analysis

Water ingress corroded BCC connector, causing communication failure

✓
After driving through standing water in the rain, the A/C stopped cooling. Scanner read DTC U011887, which would not clear. Found the BCC mounted low in the front compartment. The connector seal had aged and deformed, letting moisture enter during the water crossing. Green corrosion formed on the CAN communication pins (Pin11 and Pin22), raising contact resistance to 15Ω (normal <0.5Ω). Cleaned the pins with precision electrical contact cleaner, applied dielectric grease, installed a new waterproof seal, and wrapped the connector with waterproof tape. Resistance read normal after reconnecting; fault cleared.
BYD DTC AI Analysis

Air conditioning subnet termination resistor missing after accident repair

✓
Following front-end collision repairs, a Qin EV300 displayed multiple communication faults including U011887 and U012E87 (lost communication with electronic fan). After replacing the front compartment wiring harness, the technician failed to connect the air-conditioning sub-network twisted pair correctly and did not replace the 120Ω termination resistor at the BCC that was damaged in the accident. CAN resistance measured infinite, confirming the missing termination resistor. Replacing the BCC (with built-in termination resistor) restored total resistance to 60Ω, cleared all communication fault codes, and returned air-conditioning and thermal management functions to normal.
BYD DTC AI Analysis

Poor contact at BCC power fuse holder

✓
The vehicle failed to start after DC fast charging, with DTC U011887 and BMS-related faults stored. Inspection found the BCC power supply fuse base (front compartment fuse box F1/16 15A) had contact spring fatigue from repeated insertion and removal, causing excessive contact resistance (measured 8Ω). During high-current fast charging, this dropped the BCC supply voltage below 9V, interrupting communication. Replaced the fuse box base and fuse; supply voltage now stable at 12.4V. Fault eliminated.
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.