B1791 – Driver seat belt pretensioner 2 not connected (BYD)

B1791 means the driver seat belt pretensioner circuit is not connected on your 2020 BYD Dolphin. In plain terms, the car may not tighten the driver’s belt in a crash. You will also get an SRS warning, and the system may disable part of the restraint strategy. According to BYD factory diagnostic data, this code indicates “Driver seat belt pretensioner 2 not connected.” This is a manufacturer-specific BYD definition, so the same code number can mean something else on other platforms. Treat this as a safety-critical SRS fault. Follow BYD depowering procedures before you touch any SRS connector.

B1791 Quick Answer

B1791 on BYD points to an open or disconnected driver seat belt pretensioner 2 circuit. Diagnose the connector and harness first, with OEM-approved SRS test methods.

What Does B1791 Mean?

Official definition: “Driver seat belt pretensioner 2 not connected.” The SRS control unit sees the driver pretensioner circuit as unplugged or electrically open. In practice, the restraint system cannot rely on that pretensioner stage. The SRS warning lamp stays on to tell you the system has a fault.

What the module checks: The SRS module continuously monitors the pretensioner loop for continuity and a valid electrical signature. It does this through its internal deployment driver and sensing circuitry. Why it matters: “Not connected” usually points you toward a connector, wiring break, terminal fit issue, or an incorrect component in the circuit. Per SAE J2012-DA guidance, the DTC names a suspected trouble area. It does not prove a failed pretensioner.

Theory of Operation

Under normal operation, the BYD SRS module monitors the driver belt pretensioner circuits whenever the vehicle powers up. It expects each stage to show a valid circuit state. When a crash event meets deployment criteria, the module commands the pretensioner to fire. That action tightens the belt early in the event.

This code sets when the module no longer sees a connected circuit for “pretensioner 2.” An unplugged seat connector can trigger it in seconds. High resistance at terminals can also mimic a disconnect. Because this is an SRS pyrotechnic circuit, you must depower the SRS and follow BYD handling rules before any connector work. Do not probe these circuits with standard test leads.

Symptoms

You will usually notice a restraint warning first, then stored SRS faults on a capable scan tool.

• SRS lamp illuminated on the cluster, often staying on after start-up
• Message warning about airbag/seat belt system malfunction (wording varies)
• Stored DTC B1791 in the SRS module with “not connected” description
• Readiness SRS system may disable one or more deployment strategies
• Intermittent fault lamp and code may come and go with seat movement
• Recent work code appears after seat removal, interior repair, or collision work
• Seat movement sensitivity fault triggers when sliding the seat or adjusting height

Common Causes

• Under-seat connector partially latched: A loose lock allows intermittent open circuit on the pretensioner 2 circuit, so the SRS module sees it as “not connected.”
• Terminal fretting or corrosion at the pretensioner connector: Micro-movement under the seat raises resistance and breaks the expected circuit signature the module monitors.
• Harness damage from seat track movement: Repeated fore-aft travel can pinch, stretch, or cut the SRS wiring, creating an open circuit condition.
• Incorrect seat or belt assembly installation: A mismatch or misrouting after interior work can leave pretensioner 2 unplugged or connected to the wrong mate.
• Connector secondary lock (CPA) missing or broken: Without the secondary lock, the connector can back out over bumps and set a hard fault at key-on.
• Previous repair with non-approved terminals or splices: Improper repairs change resistance and connection integrity, and the SRS module flags the circuit as disconnected.
• Damage to clockspring/seat belt wiring path during trim work: Trim fasteners or sharp edges can cut insulation and open the circuit near the B-pillar or seat base routing.
• Internal fault in the pretensioner 2 device: An internal open in the pretensioner can mimic an unplugged connector, but you must prove wiring integrity first.

Diagnosis Steps

Use a scan tool with full BYD SRS access, wiring diagrams, and OEM-approved SRS test methods. Gather a DMM and backprobing tools for non-SRS power and ground checks only. Do not probe pretensioner circuits with standard leads. Depower and disable the SRS system using BYD procedures before you touch any yellow SRS connector.

1. Confirm B1791 in the SRS/airbag module and record all codes. Save freeze frame data if the tool supports it. Focus on battery voltage, ignition state, and any companion seat belt or crash sensor codes. Freeze frame shows conditions when the fault set. A manual scan tool snapshot helps catch intermittent opens during seat movement tests.
2. Inspect the obvious circuit path before any meter work. Check for recent seat removal, buckle work, detailing, or accessory wiring under the driver seat. Verify the pretensioner 2 connector at the seat belt assembly looks fully seated and locked. Do not disconnect SRS connectors yet.
3. Check fuses and power distribution that feed the SRS module and related restraint circuits. Use the correct fuse map for the BYD Dolphin. Verify each related fuse loads correctly. A fuse can look good and fail under load, so test both sides powered.
4. Verify SRS module power and ground integrity with voltage-drop testing under load. Keep the SRS connectors untouched. Backprobe only approved non-SRS power and ground points. Command an allowed load or observe key-on load. Confirm ground drop stays under 0.1 V while the module operates. High resistance grounds can create false “not connected” decisions.
5. Follow BYD depowering procedures and wait the specified time before disconnecting any SRS connector. Remove the key from proximity, secure the vehicle, and prevent accidental wake-up. Treat every pretensioner as a live pyrotechnic device until the system is disabled.
6. With the system depowered, disconnect and inspect the under-seat and belt-side connectors related to the driver seat belt pretensioner 2. Check for backed-out terminals, damaged seals, bent pins, or a broken secondary lock. Look for witness marks that show the connector never fully latched.
7. Inspect the harness routing from the seat belt pretensioner area through the seat base to the body harness. Pay attention to pinch points at the seat track, sharp brackets, and tie-down locations. Move the seat through full travel while watching the harness. You want to find stretching, chafing, or crushing.
8. Perform OEM-approved circuit checks for an open circuit condition. Use BYD-recommended breakout leads or test adapters, not standard meter probes in SRS terminals. Check continuity only as directed by service information. Continuity alone does not prove a good circuit under load, so prioritize connector integrity and known-good terminal fit.
9. If BYD service information provides a resistance check method using an approved simulator or substitution tool, follow it exactly. Never install resistors, jumper wires, or bypass devices as a workaround. The goal is to confirm whether the fault follows the device or stays with the wiring.
10. Reassemble connectors with correct locking steps and verify every CPA or secondary lock fully engages. Restore harness retention so the connector cannot move during seat travel. Re-enable the SRS system using BYD procedures, then clear codes with the SRS-capable scan tool.
11. Cycle ignition and recheck for immediate return. A hard open circuit monitored by the SRS module often returns at key-on if the fault remains. If the code stays cleared, run a controlled seat movement check and monitor SRS data for connector status changes. Use a scan tool snapshot during movement if the concern was intermittent.

Professional tip: On BYD platforms, seat wiring movement causes many “not connected” faults. Do not stop at a quick visual. Confirm connector latch engagement by feel and by checking terminal retention. Also prove the SRS module ground quality with a real voltage-drop test. Poor grounds create false circuit decisions and waste pretensioners.

Possible Fixes

• Reseat and correctly lock the pretensioner 2 connectors: Fully engage the connector and secondary lock, then secure the harness to prevent seat-travel pullout.
• Clean or repair connector terminals using OEM-approved methods: Correct terminal fit issues and replace damaged terminals or housings as allowed by BYD service procedures.
• Repair harness damage in the seat track or under-seat routing: Restore proper routing and protection, and use only approved SRS wiring repair practices.
• Correct misinstalled seat belt or seat components: Verify the correct mating connectors and routing after any interior or seat service.
• Replace the driver seat belt pretensioner 2 unit only after circuit proof: Replace the device if wiring, terminals, and connector retention all test good and the fault follows the pretensioner.
• Address SRS power/ground issues at the module: Repair high-resistance grounds or power feed faults that skew the module’s circuit monitoring results.

Can I Still Drive With B1791?

You can usually drive a BYD Dolphin with B1791, but you should treat the SRS system as compromised. This DTC points to the driver seat belt pretensioner 2 circuit showing “not connected.” In a crash, the pretensioner may not fire as designed. That changes occupant protection and injury risk. Do not unplug seat connectors or move harnesses with the system powered. Follow BYD SRS depowering procedures before any inspection under the seat. Use a scan tool that can access and read SRS data. Generic code readers often miss SRS details and fail to clear SRS faults correctly.

How Serious Is This Code?

B1791 is safety-critical, not a drivability fault. The car will typically start and run normally. You may only notice an airbag/SRS warning and a stored DTC. The risk sits in crash performance. A “not connected” pretensioner circuit can disable that pretensioner output. Some BYD platforms may also inhibit parts of the restraint strategy when a fault exists. Do not treat it as an inconvenience. Proper diagnosis requires SRS-safe tools, correct depowering steps, and technician training. Avoid DIY probing. Never use a standard test light or jumper wires on pretensioner circuits.

Common Misdiagnoses

Technicians often replace the buckle/pretensioner assembly too early. They skip basic connector and harness checks under the driver seat. Seat movement commonly stresses wiring and creates intermittent opens. Another common error involves testing the squib circuit with regular meter leads. That risks damaging the circuit or triggering a fault. Some shops also chase the wrong component because “pretensioner 2” varies by BYD platform. Confirm the exact circuit ID in service information. Finally, many waste time clearing codes repeatedly. B1791 will return until you correct the open connection condition.

Most Likely Fix

The most common confirmed repair path involves restoring a reliable connection at the driver seat belt pretensioner 2 connector. Focus on the under-seat connector body, terminal fit, corrosion, and harness strain relief. A close second involves repairing an open or high-resistance section in the seat-side harness where it flexes. Do not call the pretensioner failed until you verify the circuit integrity with OEM-approved SRS test methods and the correct BYD scan tool functions. After repair, run an SRS self-check and confirm no current fault returns.

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