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) — Qin Plus

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.

The B1681 fault code on the Qin Plus indicates the Passive Anti-Theft System transceiver has failed to communicate with the engine control module or recognize the key, triggering the immobilizer and preventing the engine from starting. This high-severity fault typically stems from a faulty transceiver module or a damaged transponder coil around the ignition lock cylinder. Visiting an authorised dealer is recommended for diagnosis and repair. Typical repair cost data is available based on 3 recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Internal circuit fault or poor thermal stability in the PATS transceiver module (located around the ignition lock cylinder or under the steering wheel), preventing it from generating the 125 kHz excitation signal or demodulating the 64-bit rolling code from the key chip.
  • 2Open circuit, short circuit, or abnormal resistance in the transponder coil (induction coil around the ignition lock cylinder; normal resistance approx. 0.5-1.5Ω), causing failure to energize the key chip or receive signals.
  • 3Open circuit, short circuit, or short to ground in the communication wiring between the PATS module and the PCM/ECM (usually twisted pair wires, such as gray/orange and white/light green), particularly wiring harness expansion causing poor contact after the vehicle warms up.
  • 4Key transponder damage, demagnetization, or data loss, or depleted key battery (for smart keys), resulting in failure to return the correct encrypted rolling code.
  • 5Abnormal PATS module power supply (+12V) or ground (GND) circuit, including blown fuse, oxidized or loose connector, corroded pins, or poor circuit connection.
What To Do
  • 1
    Initial diagnosis: Read the fault code with the diagnostic tool and confirm if B1681 clears or reappears immediately; observe the immobilizer light status 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 measure the voltage drop on the constant power (+12V) and ground (GND) circuits. Verify connectors are free of oxidation and looseness. Check the continuity and insulation of the communication twisted pair between the module and the PCM. Focus on wiring harness wear where the harness passes through the firewall and near the steering column.
  • 3
    Signal waveform check: Use an oscilloscope to test both ends of the transceiver coil. Inserting the key should display an RF excitation signal of approximately 125 kHz. Test the data communication line between the PATS module and the PCM (e.g., gray/orange wire). Normal operation shows pulse signal exchanges pulling 12 V low to ground. A constant 12 V high level indicates the module is not responding.
  • 4
    Component replacement verification: Perform a substitution 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 registration programming (Key Programming) and anti-theft system synchronization to match the new module with 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 Qin Plus 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.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.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.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.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 Qin Plus →Qin Plus 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.