B1640 – Driver seatbelt pretensioner not connected (BYD)

B1640 means the driver seatbelt pretensioner circuit is being seen as “not connected,” so the car may disable part of the restraint system and turn on the SRS/airbag warning. In real terms, the driver’s seatbelt pretensioner may not fire as designed in a crash, and the vehicle will usually store a hard fault until the issue is corrected. This is a manufacturer-specific BYD code, and its exact logic can vary by platform and model year. According to BYD factory diagnostic data, this code indicates the Driver seatbelt pretensioner not connected condition. Treat this as safety-critical: depower/disable the SRS per BYD procedure before touching any related connectors or wiring.

B1640 Quick Answer

On a 2020 BYD Dolphin, B1640 points to the driver seatbelt pretensioner circuit being detected as disconnected (open circuit/high resistance) by the SRS system, which can disable pretensioner deployment and illuminate the airbag/SRS warning.

What Does B1640 Mean?

Official definition: Driver seatbelt pretensioner not connected. What the module detected: the SRS/airbag control system determined the driver pretensioner circuit does not look electrically present/attached. What that means in practice: the system cannot guarantee pretensioner operation during a collision event, so it flags a fault and typically disables that output channel to prevent unintended deployment.

What the module is actually checking: the SRS module performs continuous electrical supervision of each pyrotechnic squib circuit (including the driver pretensioner) by monitoring circuit integrity (open/short/high resistance behavior) through its internal diagnostics, often at the module connector and through the seat harness path. Why that matters for diagnosis: “not connected” is a circuit integrity result, not a confirmed failed pretensioner; the root cause is often a connector/harness issue under the driver seat or an incorrect/unsafe test method that damaged terminals or shorting bars—so circuit verification and connector inspection come before any parts replacement.

Theory of Operation

Under normal operation on BYD vehicles, the SRS system monitors crash sensors and controls pyrotechnic devices such as airbags and seatbelt pretensioners. The driver seatbelt pretensioner contains an ignition element (squib) that, when commanded, rapidly tensions the seatbelt to reduce occupant movement early in a crash. The SRS module constantly supervises the pretensioner circuit so it can detect wiring faults long before a collision occurs.

B1640 sets when the SRS module’s supervision test indicates the driver pretensioner circuit is electrically “missing” or not plausible—most commonly an open circuit or excessive resistance between the module and the pretensioner. Typical failure points are the under-seat connectors, harness routing/strain from seat movement, backed-out terminals, or damage from prior seat removal. Because this is an SRS circuit, only OEM-approved test methods should be used; probing with standard test leads or applying external power can create new faults or cause an unintended deployment.

Symptoms

These are the most common signs you’ll notice when B1640 is present on a BYD Dolphin.

• SRS warning Airbag/SRS indicator illuminated and stays on
• Message Safety restraint/airbag system warning shown in the instrument cluster (wording varies by BYD software)
• No ready state SRS system may not report “normal/ready” status in scan tool data
• Pretensioner disabled Driver pretensioner output may be inhibited while the fault is active
• Stored fault Code returns immediately after clearing if the circuit is still open/disconnected
• Intermittent light Warning may flicker with seat movement if the under-seat connection is marginal
• Related DTCs Additional SRS codes may set for driver seat components sharing the same under-seat harness routing

Common Causes

• Seatbelt pretensioner connector not fully latched: A partially seated or unlocked connector at the driver pretensioner or under-seat junction creates an open-circuit condition the SRS interprets as “not connected.”
• Under-seat harness damage from seat travel: Repeated forward/back seat movement can pull, pinch, or chafe the pretensioner wiring, intermittently opening the circuit or creating high resistance.
• Terminal spread or poor pin fit at SRS connectors: Slightly widened terminals can pass a light continuity check but fail under the module’s monitoring current, setting a disconnect-type DTC.
• Corrosion or contamination in connector cavities: Moisture intrusion (wet carpet, drink spill, or cleaning) increases resistance at the terminals and can mimic an unplugged pretensioner.
• Incorrect seat or pretensioner installed (mismatch): BYD platform variations can use different pretensioner connectors or internal characteristics; a mismatched component can be seen by the module as not connected.
• Recent seat removal/airbag work without proper SRS handling: If the seat was removed or serviced and the SRS was re-enabled before all connectors were secured, the module may store a hard fault that returns immediately at key-on.
• Open circuit in the pretensioner loop wiring (vehicle side): A broken conductor inside insulation, especially near bend points and tie-downs, causes an open that the SRS monitors as a disconnected device.
• Fault in the SRS wiring junction/connector between seat and body harness: Depending on BYD build, an intermediate connector or junction can be the failure point even when the pretensioner connector itself looks fine.

Diagnosis Steps

Tools: a scan tool with full BYD SRS access (read/clear SRS DTCs, view data, and run guided functions if available), OEM service information for the Dolphin, a quality DMM, and approved SRS-safe test methods/adapters (do not probe SRS terminals with standard meter leads). You will also need basic trim tools, lighting, and the ability to perform voltage-drop testing under load.

1. Confirm the DTC and document conditions: Using a scan tool that can access the BYD SRS/airbag system, read all SRS codes and note whether B1640 is stored/confirmed or intermittent/history. Record freeze frame (if provided by the module) focusing on battery voltage, ignition state (OFF/ACC/ON), and any flags showing “pretensioner connected/not connected” or seatbelt status at the moment the fault set. Freeze frame shows the conditions when the DTC set; a scan-tool snapshot is different and can be used later to catch an intermittent during seat movement testing.
2. Check power distribution and perform a quick visual circuit-path inspection before any meter work: Verify the vehicle’s relevant SRS/airbag fuses and power feeds are intact (do not assume because the car powers up). Then inspect the most common physical failure zone: under the driver seat. Look for a harness pulled tight, crushed by seat tracks, rubbed-through loom, or a connector hanging or out of its clip. Do not disconnect anything yet; the goal is to spot obvious mechanical causes first.
3. Depower the SRS using OEM procedure before touching connectors: Follow BYD’s specified SRS disable process (ignition state, 12V power disconnect method, and required wait time). This is safety-critical: pretensioners are pyrotechnic devices. Do not use ohmmeters across pretensioner pins and do not backprobe SRS connectors with standard leads. If you are not equipped with OEM-approved SRS test adapters and procedures, stop here and refer to a qualified shop.
4. Verify SRS module power and ground integrity under load: If service information indicates shared power/ground concerns or multiple SRS DTCs are present, verify the SRS control module power and ground circuits using voltage-drop testing with the circuit operating. Ground voltage drop should be less than 0.1 V under load. This step prevents misdiagnosing a pretensioner “not connected” when the root cause is a weak ground or power feed that collapses only under load.
5. Inspect and re-seat the driver pretensioner circuit connectors (seat side and body side): With the system depowered, locate the driver seatbelt pretensioner connector(s) and any intermediate under-seat connector between the seat harness and body harness. Inspect for broken locks, bent pins, pushed-back terminals, contamination, or evidence of overheating. Correctly re-seat and latch connectors, ensuring CPA/secondary locks (if equipped) are fully engaged. Re-secure the harness to its clips so seat movement cannot tug on the connector.
6. Harness routing and stress test (mechanical verification): Before reconnecting power, confirm the harness has adequate slack through the full seat travel range and is not routed where it can be pinched by tracks. After re-enabling the system (per OEM steps), use the scan tool to watch relevant SRS data PIDs (if available) while slowly moving the seat through its range. If the concern is intermittent, use a scan-tool snapshot (manual recording) during seat movement; freeze frame won’t change, but a snapshot can capture a momentary disconnect.
7. Perform circuit integrity checks using OEM-approved methods (do not measure pretensioner resistance directly unless BYD specifies an approved simulator/adapter): If B1640 returns, follow service information for the proper method to test the pretensioner loop from the module side. On many systems, this involves checking the harness side for opens/high resistance using approved breakout leads or an OEM simulator at the correct connector, never probing the pyrotechnic device directly. The objective is to separate: (a) harness/connector open or high resistance, (b) connector terminal fit issue, or (c) the pretensioner assembly not being correctly recognized.
8. Check for terminal fit issues and intermittent opens: If the fault is position-dependent (seat movement changes status), focus on terminal tension and conductor breaks near the connector strain relief and bend points. Perform a gentle pull test on individual wires at the back of the connector (no forceful yanking) and inspect for insulation stretch or prior repairs. Any evidence of a prior splice in the SRS circuit should be treated with suspicion and corrected per OEM wiring repair standards.
9. Evaluate related DTCs and plausibility: If other SRS codes are present (for example, additional pretensioner, side airbag, or seat occupancy related faults), diagnose shared causes first (common connectors, shared grounds, water intrusion, harness damage). A single isolated B1640 strongly points to the driver pretensioner circuit path; multiple seat-related codes often indicate an under-seat harness/junction issue or power/ground problem.
10. Confirm the repair without “clear-and-hope” tactics: After corrective action, re-enable the SRS correctly, clear SRS codes with the proper scan tool, and cycle the ignition as directed by BYD. Verify B1640 does not immediately return at key-on (a hard fault typically returns quickly if still present). Then perform a controlled road test and seat movement check while monitoring SRS data. Re-scan to confirm no stored/pending SRS faults remain.

Professional tip: A “pretensioner not connected” fault on the BYD Dolphin is most often proven at the connector and harness level, not by condemning the pretensioner or the SRS module. Treat every under-seat SRS connector as a high-risk failure point: secure latching, terminal tension, and harness slack through full seat travel will solve more repeat comebacks than parts swapping. Always use SRS-approved adapters; standard meter probes can damage terminals and create the very intermittent you’re chasing.

Possible Fixes

• Properly latch/re-seat the driver pretensioner and under-seat connectors: Restore correct connector engagement, secondary locks, and harness retention after depowering the SRS per BYD procedure.
• Repair harness damage in the under-seat pretensioner circuit: Correct chafed/pinched wiring, broken conductors, or prior improper repairs using OEM-approved SRS wiring repair methods and routing/clip placement.
• Correct terminal/pin fit issues: Replace/repair affected connector housings or terminals that are spread, pushed back, or corroded, then verify stable connection during seat travel.
• Remove contamination and address water intrusion: Clean/replace contaminated connectors as required and fix the source of moisture to prevent recurrence.
• Install the correct BYD-matched component only after circuit verification: If testing proves the harness and connectors are correct and the device is not recognized, replace the driver seatbelt pretensioner/seatbelt assembly as specified for the exact Dolphin variant.
• Re-secure and re-route the harness to prevent repeat failures: Add proper slack, restore protective loom, and ensure the harness cannot contact seat tracks or sharp edges through the full range of motion.

Can I Still Drive With B1640?

You may be able to drive the 2020 BYD Dolphin normally with B1640 stored, because this fault is in the SRS restraint system rather than powertrain control. However, you should treat the Supplemental Restraint System as potentially compromised: if the driver seatbelt pretensioner is “not connected,” it may not fire as designed in a collision, and the SRS warning lamp usually indicates the system has disabled one or more deployment functions as a safety measure. Do not attempt DIY probing or connector checks under the seat with the system powered; follow BYD SRS depowering procedures and use OEM-approved methods only. Schedule diagnosis immediately, especially if the vehicle is regularly carrying passengers.

How Serious Is This Code?

B1640 is safety-critical. When the SRS module (airbag/RCM) detects the driver seatbelt pretensioner not connected, it is identifying an electrical integrity problem in a pyrotechnic device circuit (or its related wiring/connectors). This is not “just a light” and it is not an inconvenience code: the vehicle may still drive and brake normally, but occupant protection can be reduced because pretensioner operation is part of the overall restraint strategy. Diagnosis requires SRS-capable scan tooling that can read BYD restraint data and perform post-repair checks, plus technician training in SRS handling. Do not back-probe squib circuits with standard meters or apply power/ohms tests unless BYD procedures specifically allow a dedicated restraint simulator or approved test equipment.

Common Misdiagnoses

The most common mistake with B1640 on BYD vehicles is replacing the pretensioner or belt assembly based only on the wording “not connected,” without proving the circuit fault first. In practice, the cause is frequently connector related: an under-seat connector not fully latched after seat movement, a poor terminal fit, or harness strain at the seat track. Another frequent error is using a generic scan tool that cannot access SRS live data and then guessing; that leads to missed freeze-frame/event data and improper clearing attempts. Technicians also waste time chasing battery voltage or a body control module issue, when the RCM is specifically reporting an open/high-resistance condition in the pretensioner circuit path that must be verified with OEM-approved methods.

Most Likely Fix

The most frequently confirmed repair direction for B1640 is restoring proper connection integrity at the driver seatbelt pretensioner circuit: depower the SRS per BYD procedure, then inspect and correct the under-seat/seatbelt pretensioner connector engagement, terminal condition, and harness routing at the seat track for tension, pinched sections, or intermittent opens. If wiring checks and connector tension tests prove good but the fault persists and SRS data still shows an open/not connected status, the next most common confirmed direction is repair of a damaged harness section or replacement of the affected pretensioner component only after circuit verification indicates the device side is open (using BYD-approved test methods, not improvised resistance checks).

Repair Costs

SRS/airbag repair costs vary significantly by component. Diagnosis must be performed by a qualified technician with SRS-capable equipment. Do not attempt airbag system repairs without proper training and safety procedures.

Last updated: March 29, 2026