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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) — Atto 3

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
Real-World Cases
BYD DTC AI Analysis

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

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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

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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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.