B1901 – Pretensionersquib (right) circuit open (Toyota)

B1901 means the right seat belt pretensioner circuit has an electrical “open,” so the SRS may not protect you correctly in a crash. In plain terms, the airbag light usually turns on and the system may disable that pretensioner until you fix the fault. This is a Toyota manufacturer-specific code, and Toyota can define it differently by platform. According to Toyota factory diagnostic data for this DTC title, B1901 indicates a “Pretensionersquib (right) circuit open.” Treat this as a safety-critical SRS fault. Depower the SRS using Toyota procedures before you touch any yellow connectors or related wiring.

B1901 Quick Answer

B1901 on Toyota points to an open circuit in the right pretensioner squib wiring path. The SRS ECU detects the open and turns the airbag warning on because it cannot guarantee pretensioner deployment.

What Does B1901 Mean?

Official definition: “Pretensionersquib (right) circuit open.” In practice, the SRS ECU (airbag ECU) believes it cannot electrically “see” the right pretensioner igniter circuit. That matters because an open circuit can prevent the pretensioner from firing during a collision event.

What the module checks: The SRS ECU monitors the pretensioner squib circuit for continuity and plausibility using internal diagnostics. It expects a stable electrical path through the pretensioner and its harness. Why that matters for diagnosis: The code does not prove the pretensioner failed. It only points to an open condition somewhere in the circuit, including connectors, wiring, terminal fit, or a safety shorting mechanism inside connectors.

Theory of Operation

On Toyota SRS systems, the pretensioner uses a pyrotechnic squib to retract the seat belt in a crash. The SRS ECU commands deployment and constantly supervises the circuit in the background. It uses dedicated wiring and keyed SRS connectors to reduce accidental deployment risk.

B1901 sets when the SRS ECU cannot confirm continuity on the right pretensioner squib circuit. A loose seat connector, damaged harness under the seat, or poor terminal contact often causes the open. A deployed pretensioner, wrong component, or connector shorting bar that does not release can also break the expected circuit path.

Symptoms

You will usually notice an SRS warning first, then loss of protection for the affected pretensioner.

• Airbag light illuminated (SRS/MIL for restraint system)
• Stored DTC B1901 present in the SRS ECU with freeze frame or event data (tool-dependent)
• SRS readiness system not “ready,” with restraints operating in a degraded mode
• Intermittent fault warning comes and goes with seat movement or after adjusting the seat
• Seat connector sensitivity symptom appears after interior work, detailing, or seat removal
• Related codes other pretensioner/airbag circuit codes may appear if the same under-seat harness gets disturbed

Common Causes

• Dislodged right pretensioner connector: Seat movement or prior service can partially unseat the connector and open the squib circuit.
• High resistance or open at connector terminals: Poor terminal tension, fretting, or contamination increases resistance until the SRS ECU interprets the circuit as open.
• Harness damage under the seat: The right front seat harness can chafe, stretch, or pinch on the seat track and break a conductor.
• Aftermarket seat covers or accessories interference: Added hardware can pull on the SRS wiring and strain the pretensioner circuit during seat travel.
• Previous collision repair or interior work: Improper routing or missed clips leaves the harness vulnerable to repeat opens when the cabin flexes.
• Right pretensioner squib internal open: The pretensioner igniter element can open internally, but you must prove wiring integrity first.
• Shorting bar not engaging in the SRS connector: Toyota SRS connectors use internal shorting features, and damage can prevent correct contact alignment and create an open reading.
• SRS ECU connector issue on the pretensioner channel: A backed-out terminal or corrosion at the SRS ECU connector can open the circuit path without any seat-area damage.

Diagnosis Steps

Use a scan tool with full Toyota SRS access, not a generic code reader. Gather OEM wiring information, backprobe tools approved for SRS work, and a quality DVOM. Follow Toyota SRS depower procedures before touching SRS connectors. Do not probe squib circuits with standard test leads. Use an approved simulator or substitute load only if Toyota procedures allow it.

1. Confirm DTC B1901 with the SRS ECU on the scan tool. Record DTC status (current, history) and note any companion SRS codes. Review freeze frame data and focus on battery voltage, ignition state, and time since key-on when the code set. Freeze frame shows the conditions at the moment of detection. Use a scan tool snapshot later to capture an intermittent open during seat movement.
2. Perform a fast visual inspection before any meter work. Look for under-seat harness strain, crushed loom, missing retainers, and anything contacting the seat track. Check for evidence of prior seat removal or collision work. Stop and follow Toyota depower steps if you plan to touch any SRS connector.
3. Check fuses and SRS power distribution next. Verify the fuses that feed the SRS system and any related IG power circuits. Inspect fuse seating and signs of heat. Do not assume a good fuse from visual inspection alone. Confirm correct power delivery to the SRS system before testing at the ECU.
4. Verify SRS ECU power and ground integrity under load. Keep connectors secured and use service-approved access points. Perform voltage-drop testing on SRS ECU grounds with the circuit operating. Target less than 0.1V drop. A weak ground can pass continuity checks but fail under load and trigger circuit interpretation faults.
5. Depower the SRS system using Toyota’s specified procedure and waiting time. Do not skip this step. Disconnecting SRS connectors while powered risks deployment and can damage the SRS ECU. Once depowered, avoid static discharge and keep your body grounded when handling connectors.
6. Inspect the right pretensioner circuit connectors and terminals. Focus on the under-seat SRS connectors and any intermediate connectors in the harness path. Look for backed-out pins, spread terminals, corrosion, water intrusion, or damaged locks. Confirm the connector keying and CPA lock function. Do not use dielectric grease unless Toyota specifies it for that connector.
7. Check the harness routing and perform a controlled wiggle test. Reconnect non-squib connectors as needed per Toyota procedures and keep the system depowered if you must separate SRS connectors. Move the seat through its travel range while observing harness tension. Use the scan tool snapshot after re-powering to see if the DTC status changes with seat movement. An open that appears only at certain seat positions points to a broken conductor or terminal tension issue.
8. Perform circuit integrity testing using OEM-approved methods only. Do not measure resistance directly across a live squib. Use Toyota procedures for checking the pretensioner circuit from the SRS ECU side, often with an approved simulator or specified test harness. Confirm you have continuity on both circuit legs end-to-end and no intermittent opens while flexing the harness. If Toyota provides a shorting bar verification step, follow it exactly.
9. Isolate the fault by sectioning the circuit. If the wiring tests good from the ECU connector to the under-seat connector, shift focus to the short sub-harness and pretensioner side. If the fault follows the harness when you manipulate it, repair the harness. If the wiring remains stable but the ECU still flags an open with an approved simulator in place, suspect an ECU connector terminal fit issue or an ECU channel issue only after all external checks pass.
10. Confirm the repair without clearing codes as a “test.” Reassemble connectors, route and secure the harness correctly, and restore SRS power following Toyota procedures. Use the scan tool to verify DTC status and perform any required SRS initialization or zero-point procedures if applicable to the platform. Verify the warning lamp behavior and recheck for returning codes after a key cycle and a complete self-check.

Professional tip: Most repeat B1901 comebacks come from under-seat terminal fit and harness strain. Do not “ohm out” the pretensioner with a meter. Use Toyota-approved check methods and pay attention to connector lock engagement. A connector that clicks but does not lock can open again after the first seat adjustment.

Possible Fixes

• Reseat and correctly lock the right pretensioner and under-seat SRS connectors: Restore full terminal engagement and verify the CPA and latch hold under light pull.
• Repair or replace the damaged seat-side harness section: Fix chafed, pinched, or broken conductors and restore proper routing and retaining clips.
• Terminal repair at affected connectors: Replace spread, corroded, or backed-out terminals using the correct Toyota repair parts and crimp tools.
• Correct harness routing and strain relief: Reinstall clips and protect the loom so seat travel cannot pull on the SRS wiring.
• Replace the right pretensioner only after circuit proof: Confirm wiring integrity first, then replace the pretensioner if it fails an OEM-approved simulator or substitution test.
• Address SRS ECU connector issues after external checks pass: Repair ECU connector terminal fit or wiring faults before considering any module replacement.

Can I Still Drive With B1901?

You can usually drive a 2015 Toyota Auris with B1901, but you should treat the SRS system as compromised. This DTC means the SRS ECU sees an open circuit in the right pretensioner squib circuit. The car will drive normally, yet crash protection may not. The SRS warning lamp will typically stay on, and the system may disable one or more deployment functions. Do not probe SRS connectors or wiring with standard test leads. Do not unplug seat or pretensioner connectors with the system powered. Schedule diagnosis promptly with a scan tool that can access Toyota SRS data and freeze frame.

How Serious Is This Code?

This code rates as safety-critical, not a drivability problem. B1901 points to a fault in the right seat belt pretensioner ignition circuit. In normal driving, you may notice no change except the SRS lamp. In a collision, the right pretensioner may not fire as designed. That increases injury risk for that seating position. Treat the entire SRS as potentially compromised until proven otherwise. Proper diagnosis requires OEM-approved SRS procedures, correct depowering steps, and SRS-safe test methods. DIY repair creates real deployment risk and often causes new SRS faults.

Common Misdiagnoses

Technicians often replace the right pretensioner or seat belt assembly without proving an open circuit. That wastes money when the real issue sits in a connector, harness, or seat-track damage point. Another common error involves disconnecting yellow SRS connectors with the battery connected. That can set additional codes and create intermittent faults. Some shops also use a standard ohmmeter across squib circuits. That risks triggering deployment or damaging the SRS ECU’s driver circuit. Avoid guessing based on “right side” alone. Confirm circuit continuity with OEM-approved tools, verify connector lock and terminal tension, and use the scan tool’s SRS data to pinpoint the affected circuit path.

Most Likely Fix

The most frequent confirmed repair direction for B1901 involves correcting a connection problem in the right pretensioner squib circuit, not replacing parts first. Focus on the right seat area and any intermediate connectors that pass through the seat track or under-seat harness routing. Look for backed-out terminals, poor terminal tension, corrosion, or harness damage from seat movement. If wiring checks good using approved methods, then suspect the pretensioner component or an internal SRS ECU driver issue. Verify each step with a full SRS-capable scan tool before final assembly and code clearing.

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 3, 2026