B1907 – Left pretensioner squib circuit short to ground (Lexus)

Lexus-specific code — factory diagnostic data

B1907 means the SRS system has detected a fault that can disable part of the seat belt restraint protection. In real life, you will usually see the airbag or SRS warning light and the system may not deploy as designed in a crash. This is a Lexus manufacturer-specific code, so the exact logic can vary by platform. According to Lexus factory diagnostic data, this code indicates a left pretensioner squib circuit short to ground. On a Lexus CT200h, treat it as a safety-critical circuit short until you prove otherwise with correct SRS-safe testing.

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B1907 Quick Answer

B1907 on Lexus points to a short-to-ground condition in the left seat belt pretensioner squib circuit. The SRS module shuts down that deployment path and turns the SRS warning on until you correct the circuit fault and clear codes with a full SRS-capable scan tool.

What Does B1907 Mean?

Official definition: “Left pretensioner squib circuit short to ground.” The SRS airbag module sets this code when it believes the left pretensioner igniter circuit can no longer carry a controlled deployment current. In practice, that means the left seat belt pretensioner may not fire in a collision, and the SRS system will flag a fault.

What the module checks and why it matters: The SRS ECU continuously monitors the pretensioner squib circuit for electrical plausibility. It looks for an abnormally low circuit resistance or a low-voltage condition that matches a short to chassis ground. That matters because a true short can prevent proper deployment or create an unsafe electrical condition. For diagnosis, the code points to a suspected trouble area in the circuit. It does not prove the pretensioner, wiring, or SRS ECU has failed.

Theory of Operation

Under normal operation, the Lexus SRS ECU supervises each squib circuit, including the left pretensioner. It uses internal monitoring to confirm the circuit stays within an expected electrical window. When a crash meets deployment criteria, the ECU commands current to the pretensioner igniter to tighten the belt.

With B1907, the ECU sees the left pretensioner squib circuit pulled toward ground. Damage in the harness, a shorted connector, or an internal short in the component can create that path. Because SRS circuits require strict test methods, you must depower the SRS system and follow Lexus procedures before touching connectors. Avoid back-probing with standard leads or using a test light on any squib wiring.

Symptoms

These are the most common signs you will notice with B1907 on a Lexus CT200h.

• SRS warning light stays on (AIRBAG/SRS indicator)
• Disabled protection for the left pretensioner circuit until the fault clears
• Stored DTC B1907 in the SRS airbag module with a capable scan tool
• No readiness SRS system may report a fault status or incomplete self-check
• Intermittent light if the short occurs only when the seat moves or bumps happen
• Related codes other left-side squib or buckle/seat wiring codes may appear
• Post-service onset light appears after seat removal, interior work, or harness disturbance

Common Causes

• Chafed pretensioner squib harness to body ground: Insulation damage lets the squib circuit contact metal, and the SRS ECU interprets it as a short to ground.
• Corrosion or moisture in an SRS connector: Moisture bridges terminals or creates conductive corrosion paths that pull the circuit toward ground.
• Connector not fully seated or terminal back-out: A loose terminal can touch the connector shell or an adjacent grounded shield, creating an intermittent ground short.
• Damaged harness under the seat or along the B-pillar route: Seat movement and track contact can pinch the harness and cut insulation on Lexus CT200h routing points.
• Improper aftermarket wiring near SRS harness: Added equipment can pierce or tie into the wrong loom and introduce an unintended ground path.
• Previous collision or interior repair disturbance: Trim, seat, or belt work can stress the pretensioner wiring and create a hidden short point.
• Faulty left pretensioner assembly (internal short): An internal squib or lead-frame defect can short the circuit to the pretensioner housing ground.
• Shorted clock spring or nearby SRS branch harness (shared routing): Harness branches can share clips and pass-throughs, so a short in a nearby leg can mimic a pretensioner circuit short.

Diagnosis Steps

Use a scan tool with full Lexus SRS airbag access, plus a DVOM with min/max capture. Have OEM-approved SRS connector tools and back-probing adapters made for SRS terminals. Follow Lexus depowering procedures before touching any SRS connector. Do not probe squib circuits with standard test leads or powered test lights.

1. Confirm DTC B1907 in the SRS airbag module and record all SRS codes. Save freeze frame data, especially battery voltage, ignition state, and any recorded DTC status at set time. Freeze frame shows the conditions when the SRS ECU detected the short. Use a scan tool snapshot later to catch intermittent shorts during seat movement tests.
2. Perform a visual inspection of the left pretensioner squib circuit path before any meter work. Depower the SRS system using Lexus procedures, then inspect harness routing at the seat base, seat track areas, B-pillar, and any clip points. Look for pinch marks, rubbed insulation, or signs of a previous repair.
3. Check fuses and power distribution that feed the SRS system. Verify fuse condition and terminal tension at the fuse box, not just continuity. A power feed issue can confuse diagnosis and create misleading circuit behavior during self-check.
4. Verify SRS ECU power and grounds with voltage-drop testing under load. Command SRS module wake-up with the scan tool where applicable, then measure ground drop with the circuit operating. Keep ground drop below 0.1V under load. A weak ground can alter circuit bias and cause false circuit fault interpretation.
5. With SRS still depowered, disconnect the involved connectors per Lexus service information. Focus on the left pretensioner connector and the intermediate connectors in that leg. Inspect for corrosion, moisture, bent pins, terminal spread, or terminal push-out. Check CPA locks and secondary locks for full engagement.
6. Check the harness side for a short-to-ground condition without applying power to the squib. Keep the pretensioner disconnected, then measure resistance from each squib circuit wire to a known good chassis ground. You should not see a low-resistance path. If resistance drops when you move the seat or flex the loom, you found an intermittent contact point.
7. Isolate the short by splitting the circuit at intermediate connectors. Repeat the short-to-ground check on each harness segment until the short disappears. This step prevents unnecessary pretensioner replacement and narrows the fault to a specific section of loom or connector.
8. Inspect suspected chafe points closely and verify with a wiggle test. Move the seat through its full range while observing meter readings and watching scan tool data for DTC status changes. Use a scan tool snapshot during this test. A snapshot captures the exact moment the short returns, which freeze frame cannot do.
9. If the harness tests clean, evaluate the pretensioner assembly as the remaining suspect. Keep to OEM-approved methods only. Replace parts only after the wiring and connectors pass all short-to-ground isolation checks and terminal inspections.
10. After repairs, reassemble connectors with correct locking and routing. Restore SRS power following Lexus procedures. Clear SRS codes with a capable scan tool and run the SRS self-check. Confirm B1907 does not reset immediately at key-on, which would indicate a hard fault on a continuously monitored circuit.

Professional tip: A hard short-to-ground on a squib circuit usually returns immediately at key-on after the SRS self-check. If B1907 behaves intermittently, focus on seat travel, harness clips, and connector strain relief. Do not rely on continuity checks alone. Use segmentation and short-to-ground testing with the pretensioner disconnected to avoid misleading readings and unsafe testing.

Possible Fixes

• Repair chafed wiring and restore insulation: Repair the identified harness damage, then re-route and protect the loom so it cannot contact the seat track or body.
• Clean and correct connector terminal issues: Address moisture intrusion, corrosion, terminal spread, or back-out, and confirm all locks fully engage.
• Replace the affected harness section: Replace the damaged sub-harness or body harness segment when the short location sits inside a sealed or unserviceable section.
• Replace the left pretensioner assembly only after circuit proof: Replace the pretensioner when isolation testing proves the harness and connectors do not short to ground.
• Remove improper aftermarket wiring interference: Correct any non-OEM splices, piercings, or shared grounds that introduced a ground path near the SRS loom.

Can I Still Drive With B1907?

You can usually drive the Lexus CT200h with DTC B1907 present, but you should treat it as a safety-critical fault. This code tells you the SRS airbag module detected a short to ground in the left pretensioner squib circuit. That condition can disable the pretensioner, disable related airbag functions, or command the SRS warning lamp on. The vehicle will typically run and steer normally, yet crash protection may not work as designed. Do not try to “test” the system by clearing codes or unplugging connectors. Depower the SRS using OEM procedures before any inspection, and use only approved SRS test methods and tools.

How Serious Is This Code?

B1907 ranges from “no drivability change” to “major safety concern,” depending on what the SRS module disables after it detects the short. You may only notice an SRS warning lamp, yet the real issue involves occupant restraint performance. A short-to-ground on a squib circuit can come from harness damage, water intrusion, or a connector problem, and it can affect deployment logic. Treat the SRS system as potentially compromised until you confirm the root cause and verify proper operation with a scan tool that fully supports Lexus SRS functions. This diagnosis requires SRS-safe handling, depowering steps, and technician training. Avoid DIY circuit probing on squib wiring.

Common Misdiagnoses

Technicians often replace the left seat belt pretensioner immediately because the code mentions “pretensioner squib.” That skips the most common failure points on Lexus platforms: connector contamination, harness chafe to ground, or an incorrectly seated CPA/lock at the seat connector. Another frequent mistake involves using a standard multimeter or test light on squib circuits. That can set more DTCs or create a safety risk. Shops also misread the fault as a “bad SRS ECU” when the module simply reports a circuit short. Avoid guesswork by depowering the SRS first, then verifying the harness routing, connector condition, and short-to-ground on the body side using OEM-approved methods.

Most Likely Fix

The most frequently confirmed repair direction for B1907 on a Lexus CT200h involves correcting a short-to-ground in the left pretensioner squib circuit caused by harness damage near the seat track or at a stressed connector. That typically means repairing rubbed-through insulation, restoring correct connector locking, or addressing corrosion or moisture intrusion at the seat-side connectors. Do not treat the pretensioner assembly as failed until you prove the short exists in the component or its pigtail with OEM procedures. After repairs, confirm the fix by running an SRS health check and verifying the DTC stays cleared through key cycles and the system’s internal self-check sequence. Enable criteria vary by platform, so follow service information for confirmation.

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: April 9, 2026

Definition source: Lexus factory description · Autel MaxiSys Ultra & EV. Diagnostic guidance is based on factory-defined fault logic for this code.