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 8

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 8
  • 1An intermittent short circuit or poor connection in the right front wheel speed sensor wiring harness within the suspension travel range causes signal transmission resistance to fluctuate.
  • 2Metal 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.
  • 3Degraded thermal stability of the wheel speed sensor's 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., excessive clearance in the lower control arm ball joint or bearing) causes the dynamic air gap between the sensor and encoder to exceed tolerance during driving (standard is typically 0.5-1.2 mm).
  • 5Tyre specification mismatch (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 false system detection.
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 when the fault occurred. Check for 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. Verify the fixing bolt torque meets the specification (usually 8-12 N·m). Inspect the wiring harness corrugated conduit for wear, crushing, or water ingress throughout the full suspension travel.
  • 3
    Disconnect the right front wheel speed sensor connector. Measure the sensor resistance using a multimeter (electromagnetic induction type standard value: 1.0-1.5 kΩ; for Hall type, refer to the specific vehicle workshop manual). Measure the harness side power supply voltage (12 V or 5 V, depending on sensor type) and ground resistance (must be less than 1 Ω).
  • 4
    Clean the sensor tip and the magnetic encoder surface. Inspect the encoder for cracks, missing teeth, or demagnetization. Use an oscilloscope to measure the sensor dynamic output waveform. A normal waveform is a smooth sine wave (electromagnetic type) or square wave (Hall type). Amplitude must vary linearly with rotational speed, free of noise or sudden jumps.
  • 5
    After reassembly, perform the wheel speed sensor calibration procedure (if applicable). Perform 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. After confirming no abnormal signal jumps, 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 8 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.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.
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.