AU
B1681

DTC B1681 indicates the Passive Anti-Theft System (PATS) transceiver module fails to receive a valid signal or cannot establish normal communication with the engine control module (PCM/ECM) — Seal U

Safety System

DTC B1681 indicates the Passive Anti-Theft System (PATS) transceiver module fails to receive a valid signal or cannot establish normal communication with the engine control module (PCM/ECM).

This fault belongs to the Body Security system, not the SRS airbag system (Note: original documentation may misclassify the airbag switch).

The fault triggers the vehicle anti-theft immobilizer, preventing the engine from starting (No Crank/No Start).

The instrument cluster Security Light typically flashes rapidly.

The fault stems from abnormalities in key transponder recognition, 125kHz radio frequency signal transmission at the transceiver coil, or data exchange on the dedicated communication line between the PATS module and the PCM.

Consequently, the PCM cannot verify key validity and inhibits engine starting.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal circuit fault or poor thermal stability in the PATS transceiver module (located around the ignition lock cylinder or under the steering wheel), unable to generate the 125kHz excitation signal or demodulate the 64-bit rolling code from the key chip.
  • 2Transceiver coil (induction coil around the ignition lock cylinder) open circuit, short circuit, or abnormal resistance (normal resistance approx. 0.5-1.5 Ω), preventing it from energizing the key chip or receiving signals.
  • 3Open circuit, short circuit, or short to ground in the communication wiring between the PATS module and the PCM/ECM (typically a twisted pair, such as grey/orange and white/light green wires), especially poor contact due to wiring harness expansion after the vehicle warms up.
  • 4Key chip (transponder) damaged, demagnetized, or missing data, or key battery depleted (for smart keys), preventing the return of the correct encrypted rolling code.
  • 5Abnormal PATS module power supply (+12V) or ground (GND) circuit, including blown fuse, oxidized or loose connector, corroded pins, poor wiring connection, etc.
What To Do
  • 1
    Initial diagnosis: Use the diagnostic tool to read the fault code and confirm if B1681 clears or immediately returns; observe the immobilizer warning light on the instrument cluster to confirm if the vehicle has entered immobilizer lock mode; attempt to start the vehicle with the spare key to rule out a single key fault.
  • 2
    Basic circuit inspection: Check the PATS transceiver module power fuse, and the voltage drop on the constant power (+12V) and ground (GND) circuits. Verify the connector is secure and free of oxidation. Check the continuity and insulation of the communication twisted pair between the module and the PCM. Inspect the harness for wear, focusing on areas where it passes through the firewall and near the steering column.
  • 3
    Signal waveform check: Use an oscilloscope to measure across the transceiver coil. Inserting the key must generate an RF excitation signal of approximately 125kHz. Test the data communication line between the PATS module and the PCM (e.g., grey/orange wire). Normal operation displays a pulse signal exchange pulling 12V low to ground. A constant 12V high level indicates an unresponsive module.
  • 4
    Component replacement verification: Perform a replacement test using a known-good PATS transceiver module. If the fault clears, this confirms the original module is faulty. Check the PCM software version and upgrade if necessary to improve communication compatibility.
  • 5
    System rematching: After replacing the PATS transceiver module, PCM, or key, use a BYD dedicated diagnostic tool (BYD-EDS or Launch X-431 BYD dedicated version) to perform Key Programming and anti-theft system synchronization to match the new module to the vehicle anti-theft system.
Real-World Cases
BYD DTC AI Analysis

Ford PATS transceiver module communication fault (generic technical principles)

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Symptoms: No-crank condition. Immobilizer light on the dash flashing rapidly. Both factory keys failed to start the vehicle. Fault codes: B1681 (PATS Transceiver Module Signal Not Received) and B2103 (PATS Transceiver Circuit Fault). After clearing, B1681 returned immediately. Diagnosis: Checked power and ground at the PATS transceiver module – both normal. Tested the two data lines between the PATS module and PCM with an oscilloscope. The PCM was sending request signals correctly (white/light green wire switching normally), but the gray/orange wire remained stuck at 12V. The PATS transceiver was not pulling the circuit low to ground to return key information. Repair: Replaced the PATS transceiver module. The oscilloscope showed normal bidirectional communication waveforms. The vehicle started normally.
Original source ↗
BYD DTC AI Analysis

BYD PATS Module Heat-Related Failure Case

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Symptoms BYD models with engine immobiliser (Qin, Song, Tang series) exhibit intermittent no-start. The vehicle starts normally when cold, but once warm, the immobiliser warning light comes on and the starter does not crank. DTC B1681 appears intermittently. Diagnosis Measured PATS module power supply and communication circuits cold — normal. Retested hot: module supply voltage dropped from 12V to below 8V. Inspected PATS module connector under the steering column. Found oxidised and burned pins. When hot, thermal expansion increased contact resistance, starving the module of power and preventing operation. Solution Clean and repair the PATS module connector pins. If needed, add a relay to improve the power supply circuit. If the module is internally damaged, replace it and reprogram the keys.
BYD DTC AI Analysis

GM PATS system communication harness short circuit

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Symptoms: Vehicle would not start. Scan tool displayed DTC B1681; immobiliser system was locked. Diagnosis: Inspected PATS transceiver module externally – no damage found. Measured transceiver coil resistance: 1.2 Ω (normal). Checked communication wiring between the PATS module and PCM. Found damaged harness insulation at the engine bay firewall grommet, causing an intermittent short to body earth. This prevented the PCM from receiving the key signal from the PATS module. Resolution: Repaired the damaged twisted-pair communication wires. Re-insulated and fitted protective sleeving. Re-routed the harness to avoid contact with sharp metal edges. Cleared fault codes; immobiliser system communication returned to normal.
Original source ↗
Other Seal U 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.