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) — Atto 3

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.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1An intermittent short circuit or poor connection in the right front wheel speed sensor wiring harness during suspension travel causes signal transmission resistance to fluctuate.
  • 2Iron filings, oil sludge, or physical scratches on the surface of the right front wheel hub unit built-in magnetic encoder (tone ring), distorting the magnetic field modulation signal.
  • 3Deteriorated 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 system (such as excessive clearance in the lower control arm ball joint or bearing) causes the dynamic air gap between the sensor and encoder to exceed tolerance while driving (standard: 0.5-1.2 mm).
  • 5Tyre specification mismatch (e.g., installing non-original tyres) or severely abnormal tyre pressure causes actual wheel speed to deviate excessively from other wheel speeds, triggering a false system detection.
What To Do
  • 1
    Connect the VDS2000 or Launch X431 diagnostic tool, read the complete DTCs and Freeze Frame data, and 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 and visually inspect the right front wheel speed sensor installation. Confirm the retaining bolt torque meets the specification (usually 8-12 N·m). Check 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 (inductive type standard value: 1.0-1.5 kΩ; for Hall-effect type, refer to the specific vehicle repair manual). Measure the harness-side power supply voltage (12V or 5V, depending on sensor type) and ground resistance (must be less than 1 Ω).
  • 4
    Clean the sensor tip and magnetic encoder surface. Inspect the encoder for cracks, missing teeth, or demagnetization. Use an oscilloscope to measure the dynamic sensor output waveform. The normal waveform is a smooth sine wave (electromagnetic type) or square wave (Hall type). Amplitude must vary linearly with rotational speed, without noise or jumps.
  • 5
    After reassembly, perform the wheel speed sensor calibration procedure (if applicable). Road test the vehicle 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.

✓
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

✓
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

✓
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 Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.