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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) — Seal 6 EV

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.

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Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Air conditioning sub-network CAN wiring harness fault: CAN-H or CAN-L circuit open, short to ground, short to power, or poor connection in the high-temperature, high-humidity front compartment environment. This especially occurs near the right front wheel arch (common BCC mounting location) due to wiring harness corrosion from water or mud ingress.
  • 2BCC power supply or ground fault: A blown BCC constant power (+B) fuse, poor relay contact, or a loose or oxidized ground point (G point) causes 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, or water ingress corrosion on the pins (especially CAN communication pins), interrupting signal transmission.
  • 4BCC internal fault: Damaged controller internal CAN transceiver chip (TJA1042 or similar model), power management chip failure, 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 it fails.
What To Do
  • 1
    Freeze frame analysis: Use VDS2000/3000 or a BYD dedicated diagnostic tool to read the fault freeze frame. Confirm vehicle speed, ambient temperature, and air conditioning status when the fault occurred, and determine if it is an intermittent fault (e.g., occurred on rough roads or after wading).
  • 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 must be 12V±0.5V, and ground resistance must be less than 1Ω. Check the BCC-related fuse in the front compartment power distribution box (such as F1/16 15A) for a blown condition 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. After turning off the ignition switch, the resistance should be approximately 60 Ω (two 120 Ω resistors in parallel). A 120 Ω reading indicates one terminating resistor is missing. An infinite reading indicates an open circuit. A 0 Ω reading indicates a short circuit.
  • 4
    CAN line voltage and waveform check: Turn ignition switch ON. Measure CAN-H to ground voltage (2.5-2.7V) and CAN-L to ground voltage (2.3-2.5V); the difference between them 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 the firewall pass-through.
  • 6
    Substitution verification: If the wiring harness is normal, first update the BCC software to the latest version; if unsuccessful, cross-check the BCC unit by substitution (swap with a unit from the same model or install a new part), and finally verify the air conditioning controller.
  • 7
    Fault Clearing and Road Test Verification: After repair, clear the fault code. Conduct a road test of over 20km, including bumpy roads, maximum A/C cooling mode, and DC fast charging conditions. Confirm 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

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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

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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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.