AU
C1204

DTC C1204 indicates the ABS/ESP control unit detected the right front wheel speed sensor output signal change rate (acceleration/deceleration rate) exceeds the configured system threshold (typically ±8-12m/s²) — Seal U

Braking System

DTC C1204 indicates the ABS/ESP control unit detected the right front wheel speed sensor output signal change rate (acceleration/deceleration rate) exceeds the configured system threshold (typically ±8-12m/s²).

Unlike simple signal loss (C1203) or a static zero signal (C1205), this fault indicates the signal is present but exhibits severe non-linear fluctuations, sudden jumps, or sawtooth distortion.

The control unit monitors the wheel speed difference between adjacent sampling cycles to determine if the change is physically plausible (plausibility check).

Upon detecting an abnormal jump, the system deems the wheel speed signal unreliable and triggers a degraded protection mode for the ABS, ESP, TCS, EPB, and other systems.

This limits or disables related functions to ensure braking safety.

Common causes include electromagnetic interference affecting the sensor signal, dynamic gap changes between the sensor and the magnetic encoder, or intermittent failure of the magnetoresistive element inside the sensor.

The C1204 fault code on the Seal U indicates the ABS/ESP control unit has detected implausible speed changes from the right front wheel speed sensor. Severity is medium, as ABS, ESP, TCS, or EPB functions may be limited for safety. Common causes include damaged or loose sensor wiring, or contamination or damage on the wheel hub magnetic encoder. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1An intermittent short circuit or poor connection in the right front wheel speed sensor wiring harness within the suspension travel range causes dynamic changes in signal transmission resistance.
  • 2Iron filings, oil sludge, or physical scratches on the surface of the built-in magnetic encoder (tone ring) in the right front wheel hub unit, distorting the magnetic field modulation signal.
  • 3Deteriorated thermal stability of the wheel speed sensor internal magnetoresistive element (Hall IC or MR element) causes output signal drift at high temperatures.
  • 4Excessive play in the right front suspension system (e.g., lower control arm ball joint or excessive bearing clearance) causes the dynamic air gap between the sensor and encoder to exceed tolerance during driving (standard is typically 0.5-1.2mm).
  • 5Mismatched tyre specifications (e.g., installing non-OEM specification tyres) or severely abnormal tyre pressure causes the actual wheel speed to deviate significantly from other wheel speeds, triggering a system misjudgment.
What To Do
  • 1
    Connect the VDS2000 or Launch X431 diagnostic tool. Read all DTCs and Freeze Frame data. Record the vehicle speed, deceleration, and system voltage at the time of the fault. Check for related fault codes such as C1203 (open circuit) or C1205 (signal zero).
  • 2
    Raise the vehicle to a suitable height. Visually inspect the right front wheel speed sensor installation. Verify the mounting bolt torque meets the specification (usually 8-12 N·m). Inspect the wiring harness corrugated conduit for signs of wear, crushing, or water ingress throughout the full suspension travel.
  • 3
    Disconnect the right front wheel speed sensor connector. Use a multimeter to measure the sensor resistance (standard value for electromagnetic induction type: 1.0–1.5 kΩ; for Hall type, refer to the specific vehicle repair manual). Measure the power supply voltage on the wiring harness side (must be 12 V or 5 V, depending on sensor type) and the ground resistance (must be less than 1 Ω).
  • 4
    Clean the sensor sensing tip and the magnetic encoder surface. Check the encoder for cracks, missing teeth, or demagnetisation. Use an oscilloscope to measure the sensor dynamic output waveform. The waveform should be a smooth sine wave (electromagnetic type) or square wave (Hall type). Amplitude must vary linearly with rotational speed, with no noise or sudden jumps.
  • 5
    After installation, perform the wheel speed sensor calibration procedure (if applicable). Road test the vehicle and use the diagnostic tool to read the Live Data for all four wheel speeds. Compare the signal synchronization of the right front wheel with the other three wheels during constant speed, acceleration, and deceleration. Verify there are no abnormal signal jumps, then clear the fault code.
Real-World Cases
BYD DTC AI Analysis

Chafed right front wheel speed sensor wiring harness caused DTC C1204 in the BYD Yuan EV.

✓
When driving over rough surfaces, the instrument cluster occasionally displayed 'Check ESP System' and the ABS warning light illuminated intermittently. Scanned with a VDS2000 diagnostic tool and retrieved fault code C1204 (Excessive right front wheel speed sensor variation) from the ABS system. Lifted the vehicle and found the right front wheel speed sensor wiring harness had chafed through at the steering knuckle clip, causing insulation damage and an intermittent short in the internal copper wires. Repair: Replaced the right front wheel speed sensor wiring harness assembly, rerouted the harness and fitted a chafe guard, ensuring at least 5 mm clearance from moving suspension components. Cleared the fault codes and road-tested for 20 km. Live data showed smooth right front wheel speed signals with no fluctuation. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD E2: Dirty right front wheel hub magnetic encoder caused abnormal signal

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Customer reported the ESP warning light stayed on after driving in rain, with abnormal ABS intervention during braking. Diagnosis found DTC C1204. Removed the right front brake caliper and disc. Found heavy ferromagnetic dust on the magnetic encoder built into the wheel bearing unit (from brake pad wear and driving through water). The contamination distorted the magnetic field, causing periodic signal jumps at the sensor. Fix: Thoroughly cleaned the magnetic encoder with dedicated cleaner and demagnetized it. Checked brake pad wear and cleaned the brake system. After reassembly, sensor air gap measured 0.6mm (spec 0.5-1.2mm). Oscilloscope showed normal waveform. Fault has not returned.
Original source ↗
BYD DTC AI Analysis

BYD E3 C1204 fault fixed after non-genuine wheel replacement

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After replacing the right front wheel bearing unit (aftermarket part) at an external repair shop, the ABS and ESP warning lights stayed on. Read DTC C1204. Inspection revealed the aftermarket wheel hub's magnetic encoder pole count (48 poles) did not match the OEM part (52 poles), and magnetic flux density was low. This caused the sensor output signal frequency to mismatch the left front wheel, and the ABS control unit detected abnormal signal variation. Solution: Replaced the right front wheel bearing unit with an OEM part and performed the 'wheel speed sensor signal zero point calibration' procedure using VDS. After calibration, performed straight-line acceleration and emergency braking tests. Four-wheel speed data streams synchronized well, the warning lights went out, and system function returned to normal.
Original source ↗
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.