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B1728-00

DTC B1728-00 indicates the Airbag Control Unit (ACU) detects the squib circuit resistance of the right knee airbag (usually located beneath the passenger-side dashboard) exceeds the calibrated threshold — Seal U

Safety System

DTC B1728-00 indicates the Airbag Control Unit (ACU) detects the squib circuit resistance of the right knee airbag (usually located beneath the passenger-side dashboard) exceeds the calibrated threshold.

The normal range is typically 1.5-3.0 Ω, varying by vehicle calibration.

High resistance generally means >3.5 Ω or a near open-circuit condition.

This is an actively monitored SRS fault.

The ACU continuously monitors the airbag squib circuit current and resistance to determine circuit integrity.

High resistance indicates a high-resistance connection in the squib circuit.

This may result in insufficient ignition energy during a collision, preventing proper airbag deployment.

In extreme cases, the system detects an open-circuit fault and triggers a secondary lockout, compromising the entire airbag system operation.

This fault illuminates the instrument cluster SRS warning lamp and forces the vehicle passive safety system into a degraded protection mode.

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Cases Logged
Real-World Cases
BYD DTC AI Analysis

Water ingress caused oxidation of the right knee airbag connector on the BYD Qin EV300, resulting in high resistance.

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SRS warning light came on after the vehicle waded through water. Read DTC B1728-00. Inspection revealed signs of water intrusion in the front passenger footwell. The right knee airbag connector (under the HVAC duct) had a degraded seal and green copper corrosion on its pins. Contact resistance measured 5.8Ω. Cleaned the pins with electronic cleaner, applied conductive grease, and replaced the connector seal. Fault resolved. Recommend checking connector sealing condition on vehicles from the same production batch.
BYD DTC AI Analysis

Qin 80 airbag module internal resistance drift replacement case

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Customer reported the SRS light illuminating intermittently. B1728-00 was a historic fault code. Checked wiring harness connectors – no abnormalities. Verified harness continuity with a load simulator – normal. Measured old airbag module resistance: 4.2Ω (exceeds 3.0Ω upper limit). Diagnosed internal igniter degradation in the airbag module. Replaced right knee airbag module; resistance restored to 2.1Ω, fault resolved. Root cause: prolonged high-temperature operation caused oxidation of the internal bridge wire.
BYD DTC AI Analysis

Dashboard Wiring Harness Intermittent Partial Open Circuit Misdiagnosis Case

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The fault recurred intermittently after multiple repairs. The airbag module had been mistakenly replaced with no effect. Finally traced the fault to the instrument panel crossbeam mounting clip pinching the wiring harness, which had fractured 90% of the internal copper strands, leaving only minimal continuity. Resistance was normal when cold but rose to 3.8Ω when hot or under vibration, triggering the fault code. Repaired the harness and rerouted it to avoid the interference; fault resolved. Tip: These faults require a 'wiggle test' (measuring while moving the harness) to detect.
BYD DTC AI Analysis

ACU Sampling Circuit False Positive Diagnosis

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The vehicle has no accident history. Fault codes B1728-00 and B1725-00 (left knee airbag) appeared simultaneously. Both left and right knee airbag harnesses and modules tested normal, pointing to an ACU fault. Connecting the right knee airbag harness to a spare ACU channel (e.g., a reserved seat channel) transferred the fault code to that channel, confirming a damaged sampling resistor in the ACU's right knee airbag detection channel. Replacing the airbag control unit resolved the issue.
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.