AU
C055000

DTC C055000 indicates an internal self-check fault in the ABS electronic control unit (ECU) — Seal U

Braking System

DTC C055000 indicates an internal self-check fault in the ABS electronic control unit (ECU).

Specifically, the control module integrated inside the ABS hydraulic modulator assembly detects a hardware or basic software abnormality.

This constitutes a 'core-level' ABS system fault, indicating the ECU cannot execute normal anti-lock braking control logic.

When triggered, the ABS system forces Fail-Safe Mode.

This disables anti-lock braking, EBD (Electronic Brakeforce Distribution), and ESC (Electronic Stability Control) functions, retaining only conventional hydraulic braking.

The instrument cluster illuminates the ABS warning light, ESC warning light, and brake system warning light.

Some models also display the text prompt 'Please check brake system'.

This DTC differs fundamentally from wheel speed sensor signal faults (such as the C003 series).

The latter represents an 'input signal abnormality', whereas C055000 represents a 'control unit internal abnormality', involving significantly higher repair complexity and cost.

The C055000 fault code on the Seal U indicates an internal self-check fault in the ABS ECU, which can disable ABS, EBD, and ESC while leaving normal braking available. Severity is medium, but braking safety features may be reduced. Common causes include internal ABS modulator ECU damage or abnormal supply voltage from the battery or wiring. The recommended fix is to visit an authorised dealer for diagnosis. Typical repair cost data is available from recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damage to the internal ECU circuit board of the ABS hydraulic modulator assembly: Physical damage to the internal processor, memory, or power management chip causes the self-check procedure to fail.
  • 2Abnormal supply voltage: Battery voltage too low (<9V) or too high (>16V), excessive voltage drop in the power supply circuit, or voltage regulator module failure causes the ECU supply voltage to fall outside the normal operating range (normally requires 12V±0.5V).
  • 3Poor ground circuit: Oxidation, looseness, or corrosion at the ECU main ground point (usually located in the engine compartment or near the battery negative terminal) causes reference voltage drift, triggering ECU misjudgment or reset.
  • 4Internal communication bus fault: A damaged ECU internal CAN transceiver or LIN communication chip interrupts communication with the vehicle network or causes abnormal communication between internal modules.
  • 5Severe electromagnetic interference: Electromagnetic pulses from high-power aftermarket electrical equipment, non-standard LED headlights, or inverters disrupt normal ECU operation, causing intermittent fault codes.
What To Do
  • 1
    Connect the BYD dedicated diagnostic tool (VDS or ED400), read the complete DTC list and freeze frame data, check for accompanying codes such as C055500 (wheel speed sensor protocol error), and record parameters including vehicle speed and voltage at the time of the fault.
  • 2
    Check battery condition: measure static voltage (should be ≥12.4 V), cranking voltage (should be ≥9.6 V), and charging voltage (should be 13.5–14.8 V). Check battery terminals for oxidation. Perform a battery load test if necessary.
  • 3
    Check the ABS system power supply: disconnect the ABS hydraulic modulator assembly connector and measure the voltage between the power supply terminal (usually pin 1 or 25; refer to the vehicle wiring diagram) and ground. The voltage must be within 12V ± 0.5V. Check if the ABS fuse (usually 30A or 40A) is blown or has poor contact.
  • 4
    Check the ECU ground circuit: Use a multimeter to measure the resistance between the ECU ground terminals (usually pins 13 and 38) and the battery negative terminal. Resistance must be <1 Ω. Inspect ground points G101/G102 (depending on vehicle model) for cleanliness and tightness. If necessary, sand to remove rust and apply conductive grease.
  • 5
    Check for wiring short circuits: Test if the wheel speed sensor power circuit is shorted to the main power circuit (according to the BYD repair manual, the setting conditions for this fault code include 'wheel speed sensor power supply shorted to 12V detected'). Focus on wiring harness wear points.
  • 6
    Eliminate electromagnetic interference: Remove recently installed aftermarket electrical accessories (e.g., inverters, high-power audio systems, non-standard LED lights). Clear the fault code, road test the vehicle, and observe if the fault recurs.
  • 7
    Software update attempt: For certain post-2020 models supporting OTA updates or diagnostic tool flashing (such as Song PLUS DMi), update the ABS control unit software to the latest version (verify TCU version compatibility).
  • 8
    Replace the ABS hydraulic modulator assembly: If all above checks are normal and confirm a faulty internal ECU, replace the complete ABS hydraulic modulator assembly including the ECU (part number varies by model, e.g., BG-3550100-S for E5). Note: After replacement, use the diagnostic tool to perform the 'Bleeding Procedure', 'Wheel Speed Sensor Signal Check', and 'G Sensor/Yaw Rate Sensor Calibration'.
  • 9
    Final verification: Clear all DTCs. Drive straight at a speed above 20 km/h and apply the brakes multiple times. Confirm the ABS function restores and the warning lamp turns off. Read the DTCs again to confirm C055000 and related fault codes are not present.
Real-World Cases
BYD DTC AI Analysis

2019 BYD E5 ABS Pump Assembly Internal Damage Case

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Vehicle: 2019 BYD E5, 85,000 km. Symptoms: ABS and ESP warning lights suddenly lit up while driving. Instrument cluster showed "Check brake system" and brake pedal became stiff. Diagnosis: VDS scan retrieved DTC C055000 (ABS ECU fault). Freeze frame: 45 km/h, 13.8 V. Battery, fuses and ground tested normal. Power reset did not clear the code. Power and ground at ABS pump connector measured normal. Internal ECU in ABS hydraulic modulator assembly failed. Repair: Replaced ABS hydraulic modulator assembly with integrated ECU (part number BG-3550100-S). Bled brake system and calibrated wheel speed sensors. Fault cleared. Cost: Parts approx. 1800 yuan, labor 400 yuan.
BYD DTC AI Analysis

Poor ground in Song Plus DM-i causes intermittent C055000 fault

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Vehicle: 2021 BYD Song PLUS DM-i, 32,000 km. Symptoms: ABS warning light intermittently comes on when driving on rough roads; goes off after restarting. Highway driving is normal. Diagnosis: Scanner retrieved stored fault codes C055000 and C055500 (left front wheel speed sensor protocol error); no active faults present. Inspection found the ABS pump ground point G101 on the left front side member had a slightly loose mounting bolt and white oxidation on the mating surface. Vibration caused ground resistance variations, resulting in abnormal ECU supply voltage and triggering internal protective fault codes. Repair: Removed the ground wire, sanded the ground point and harness terminal to bare metal, applied conductive paste, and tightened the bolt to the specified torque of 10 N·m. Cleared the fault codes and test-drove 20 km; the fault did not return.
BYD DTC AI Analysis

E5 Taxi Battery Aging Caused ABS ECU Fault

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Vehicle: 2018 BYD E5 taxi, 240,000 km. Symptoms: ABS warning light on during cold morning starts, went out after 10 minutes of driving, but recently stayed on continuously. Diagnosis: DTC C055000 stored. Freeze frame showed 10.2V fault voltage. Measured static battery voltage at 11.8V (low). Load test showed battery capacity at only 45% of rated value with high internal resistance. Battery degradation caused voltage sag during startup, triggering ABS ECU low-voltage protection. Repair: Replaced AGM battery (12V 60Ah), cleaned ABS ground points, cleared codes. Monitored for three consecutive days—no recurrence. Note: The 12V auxiliary battery in pure electric vehicles is equally critical. Inspect and replace every 2-3 years.
BYD DTC AI Analysis

ABS ECU internal short circuit after water ingress

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Vehicle: BYD Song Pro DM, 50,000 km. Background: Drove through standing water (~40 cm deep). ABS light stayed on next day. Diagnosis: Scan tool read DTC C055000; the code would not clear. The ABS hydraulic modulator sits on the left side of the engine bay. Despite the high mounting position, water splashed into the connector during the crossing, leaving moisture inside the plug. Insulation resistance between the ABS pump power terminal and ground measured only 2 MΩ (normal >10 MΩ), indicating the internal circuit board had shorted from moisture. Resolution: Replaced the ABS hydraulic modulator assembly and the oxidized harness connector. Sealed the surrounding wiring with insulating silicone grease. Fault cleared. Recommendation: Check the ABS pump connector seal after driving through water.
BYD DTC AI Analysis

Modified inverter causing false EMI detection

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Vehicle: 2018 BYD E5, 60,000 km. Symptoms: After installing a 2000W high-power onboard inverter, the ABS warning light illuminates intermittently when the inverter is in use; the fault disappears when the inverter is switched off. Diagnosis: Retrieved DTC C055000 with no accompanying faults. Checked ABS power supply, ground, and wiring—all normal. Determined that high-frequency electromagnetic pulses generated during inverter operation interfere with the ABS ECU through power lines or radiated emissions. Solution: Removed the non-factory inverter and switched to the BYD dedicated 12V power outlet (cigarette lighter socket, maximum 120W), or added a shielding cover to the inverter with independent ground. Cleared the fault codes and tested repeatedly; the fault did not recur. Note: The ABS ECU is sensitive to electromagnetic interference; avoid high-power non-standard electrical devices.
Other Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.