AU
C1209

BYD DTC C1209 indicates the ABS system detected an electrical fault in the right rear wheel speed sensor (RR Wheel Speed Sensor) circuit, specifically an open circuit, short to ground, or short to power in the signal line — Seal U

Braking System

BYD DTC C1209 indicates the ABS system detected an electrical fault in the right rear wheel speed sensor (RR Wheel Speed Sensor) circuit, specifically an open circuit, short to ground, or short to power in the signal line.

In new energy vehicles, the wheel speed signal not only controls ABS/ESC braking but also directly affects the motor regenerative braking strategy, driving range calculation, and automatic emergency braking (AEB) function.

This fault causes a lost or abnormal right rear wheel speed signal, forcing the ABS into fail-safe mode.

This disables anti-lock braking, traction control, and vehicle stability functions, and may trigger regenerative braking derating, creating a safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Open or short circuit in right rear wheel speed sensor internal coil (sensor aging, overheating damage)
  • 2Loose wiring harness connector, backed-out terminal, or seal failure causing water ingress and corrosion (common after chassis water exposure).
  • 3Sensor wiring harness insulation damaged, causing a short to body ground or signal wire open circuit.
  • 4Incorrect sensor installation gap or tone ring damage (missing teeth, deformation, contamination) generates an abnormal signal, causing the system to misdiagnose a circuit fault.
  • 5ABS control unit internal signal processing circuit fault (less common)
What To Do
  • 1
    Use the diagnostic tool to read freeze frame data. Confirm vehicle speed and wheel speed status at the time of the fault to rule out intermittent interference.
  • 2
    Raise the vehicle. Visually inspect the right rear wheel speed sensor appearance, installation, and wiring harness routing. Check for obvious damage.
  • 3
    Disconnect the sensor connector and measure the sensor resistance (normal value approx. 1.0-1.6 kΩ at 20°C). Infinite resistance indicates an open circuit; 0 or extremely low resistance indicates a short circuit.
  • 4
    Measure wiring harness continuity between the sensor connector and the ABS control unit to check for an open circuit; measure wiring harness insulation resistance to ground and to the power supply to check for a short circuit.
  • 5
    Check if the connector pins are oxidized, backed out, or deformed. Clean or replace the connector if necessary.
  • 6
    Remove the sensor and check the sensing tip for dirt or damage. Use a feeler gauge to check the air gap between the sensor and the tone ring (standard value typically 0.3-1.2 mm).
  • 7
    Check the tone ring for missing teeth, deformation, or heavy contamination (metal filings buildup).
  • 8
    After replacing the faulty sensor or repairing the wiring harness, clear the fault code and perform a road test to verify (accelerate above 40 km/h and observe if the wheel speed data for all four wheels synchronize).
Real-World Cases
BYD DTC AI Analysis

BYD E2 right rear wheel speed sensor internal short circuit

✓
Dashboard ABS and ESC warning lights remained illuminated. Read DTC C1209. Measured right rear wheel speed sensor resistance at 0.2Ω (normal approximately 1.2kΩ), indicating internal coil short. Replaced right rear wheel speed sensor; new unit measured 1.15kΩ. Cleared fault codes after installation and road tested. All four wheel speed signals normal. Fault resolved.
BYD DTC AI Analysis

BYD Yuan EV wiring harness chafing causes intermittent open circuit

✓
While driving on rough roads, the ABS warning lamp illuminated intermittently. DTC C1209 set with intermittent status. Inspection found the RH rear wheel speed sensor harness chafed through at the underbody shield retaining clip, with copper strands fractured causing poor contact. Stripping back the harness revealed 2 of the 3 wires broken. Resoldered the connections and applied waterproof insulation. The fault did not recur.
BYD DTC AI Analysis

Connector corrosion on BYD E3 after water ingress

✓
After driving through standing water, the ABS warning light illuminated and stored DTC C1209. Inspection found water intrusion and green corrosion inside the right rear wheel speed sensor connector; oxidised pins caused poor signal contact. Cleaned the connector with electrical contact cleaner, applied conductive grease, and replaced the seal. Fault cleared. Recommend checking chassis sealing to prevent further water ingress.
BYD DTC AI Analysis

BYD E1 deformed reluctor ring causing false circuit fault

✓
Impact to the right rear wheel triggered DTC C1209 and illuminated the ABS warning light. Initial inspection found the wheel speed sensor and wiring harness normal, but the diagnostic tool showed an abnormal right rear wheel speed signal. Disassembly revealed the reluctor ring inside the brake drum had deformed from the impact, rubbing against the sensor and causing it to overheat and fail. The reluctor ring's signal teeth were also deformed. Replacing the right rear wheel bearing unit (with reluctor ring) and the wheel speed sensor resolved the fault.
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.