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 6 EV

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.

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Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Intermittent short circuit or poor connection in the right front wheel speed sensor wiring harness within the suspension travel range causes dynamic variations 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 cause magnetic field modulation signal distortion.
  • 3Degraded thermal stability of the magnetoresistive element (Hall IC or MR element) inside the wheel speed sensor body causes output signal drift at high temperatures.
  • 4Excessive play in the right front suspension (such as 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: 0.5-1.2 mm).
  • 5Mismatched tyre specifications (e.g., installing non-factory specification tyres) or severely abnormal tyre pressure causes the actual wheel speed to deviate excessively from other wheel speeds, triggering a system misjudgement.
What To Do
  • 1
    Connect the VDS2000 or Launch X431 diagnostic tool. Read the complete DTCs and freeze frame data. Record the vehicle speed, deceleration, and system voltage at the time of the fault. Check for accompanying 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 retaining 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 (electromagnetic induction type standard value: 1.0-1.5kΩ; for Hall type, refer to the specific vehicle repair manual). Measure the harness-side supply voltage (12V or 5V, depending on sensor type) and ground resistance (must be less than 1Ω).
  • 4
    Clean the sensor tip and magnetic encoder surface. Check the encoder for cracks, missing teeth, or demagnetization. Use an oscilloscope to measure the sensor's dynamic output waveform. The normal waveform is a smooth sine wave (electromagnetic type) or square wave (Hall type). The amplitude must vary linearly with rotational speed, with no noise or jumps.
  • 5
    After installation, perform the wheel speed sensor calibration procedure (if applicable). Conduct a road test and use the diagnostic tool to read the Live Data stream for all four wheel speeds. Compare the right front wheel signal synchronization with the other three wheels under constant speed, acceleration, and deceleration conditions. Confirm 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.

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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 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.