B1861 – Knee airbag (driver) squib circuit open (Toyota)

B1861 means the driver knee airbag may not deploy in a crash. You will usually see the SRS/airbag warning light stay on, and the system may disable part of the restraint protection. According to Toyota factory diagnostic data, this is a manufacturer-specific code that indicates a “knee airbag (driver) squib circuit open” on a 2015 Toyota Auris platform. “Open” points to a broken electrical path, not a confirmed bad airbag. Treat this as safety-critical. Depower the SRS using Toyota procedures before touching any related connectors or wiring. Use only OEM-approved SRS test methods.

B1861 Quick Answer

B1861 on Toyota indicates the SRS ECU sees an open circuit in the driver knee airbag inflator (squib) circuit. The airbag light stays on and the knee airbag circuit will not arm until you restore circuit continuity.

What Does B1861 Mean?

Official definition: “Knee airbag (driver) squib circuit open.” In plain terms, the SRS ECU believes the driver knee airbag igniter circuit has a break. That prevents proper monitoring and can disable deployment for that circuit. The warning light tells you the system cannot guarantee protection.

What the module checks and why it matters: The Toyota SRS ECU continuously monitors the knee airbag squib loop through dedicated wiring and connectors. It looks for expected continuity and a plausible circuit load. When the circuit goes open, the ECU logs B1861 and turns on the SRS lamp. For diagnosis, “open” means you must prove where continuity stops. Verify connectors, terminals, and harness routing before condemning the airbag module.

Theory of Operation

Under normal conditions, the Toyota SRS ECU supplies a monitored current through the driver knee airbag squib circuit. The ECU uses that feedback to confirm the circuit stays intact and ready. It also runs key-on self-checks and continuous monitoring while driving.

An “open” happens when the monitored path breaks. A loose connector, damaged terminals, or a broken harness strand can cause it. High resistance from corrosion can also mimic an open as it worsens. When the ECU cannot see the expected circuit continuity, it flags B1861 and disables that firing circuit for safety.

Symptoms

These symptoms match a Toyota SRS squib circuit open fault on the driver knee airbag circuit.

• SRS light stays on or comes back on shortly after startup
• Message “Check SRS” or similar warning on the cluster (equipment varies)
• No readiness SRS system shows a stored DTC and will not fully arm the affected circuit
• Scan results SRS ECU reports B1861 and may show the knee airbag circuit as “open” in data
• Intermittent warning that changes with seat/dash movement or temperature
• Post-repair history code appears after dash, steering column, or interior trim work near the knee area
• Inspection clues evidence of harness strain, pinched wiring, or disturbed connectors near the lower dash

Common Causes

• Disconnected knee airbag squib connector: A partially seated connector adds an open in the driver knee airbag deployment circuit.
• High resistance or open in the harness near the steering column or lower dash: Harness flex and contact with brackets can break conductors and create an intermittent or hard open.
• Terminal damage at the knee airbag module connector: Spread, backed-out, or deformed terminals stop current flow and the SRS ECU flags an open circuit.
• Corrosion or moisture intrusion at SRS connectors: Oxidation increases resistance until the ECU interprets the squib circuit as open.
• Previous interior work left an SRS connector unlocked: Knee bolster, lower panel, or column service can leave CPA locks unlatched and open the circuit.
• Incorrect parts or sub-harness after collision repair: A mismatched knee airbag, pigtail, or connector keying can prevent proper pin contact.
• Shorting bar not functioning inside the squib connector: A connector safety shorting mechanism that sticks can prevent the ECU from seeing the correct circuit path.
• SRS ECU (airbag sensor assembly) connector issue: Poor pin fit or damage at the ECU connector can open the monitored knee airbag squib loop.

Diagnosis Steps

Use a scan tool with full Toyota SRS access, plus OEM wiring diagrams and connector views for the 2015 Auris. Follow Toyota SRS depowering procedures before touching any SRS connector. Use only OEM-approved SRS test methods and adapters. Avoid standard probes and powered test lights on squib circuits. A quality DMM supports voltage-drop checks on ECU power and grounds.

1. Connect the scan tool and run a full SRS health check. Confirm B1861 status as pending, stored, or current. Record freeze frame data, especially battery voltage, ignition state, and any related SRS DTCs. Freeze frame shows conditions when the DTC set. Use a scan tool snapshot later to capture an intermittent open during a wiggle test.
2. Perform a fast visual inspection of the suspected circuit path before meter work. Inspect the lower dash and knee bolster area for recent repairs, missing fasteners, pinched harnesses, or aftermarket equipment. Check SRS-related fuses and power distribution next. Do not backprobe the SRS ECU yet.
3. Verify SRS ECU power and ground integrity under load. Keep the SRS system in a safe state per Toyota procedures. Use voltage-drop testing, not continuity alone. Command or observe an SRS-related load if the service procedure allows it. Confirm ground drop stays under 0.1 V with the circuit operating. High resistance can mimic an open squib circuit.
4. Depower the SRS system using Toyota’s specified method and wait the required time. Confirm you can safely handle SRS connectors. Do not use an ohmmeter across an airbag inflator. Do not apply power to any squib circuit.
5. Locate the driver knee airbag and its squib connector. Inspect connector seating, CPA lock engagement, and connector body damage. Look for pulled wires, strain marks, or a harness routed over sharp edges. Correct any obvious connector lock or routing issues first.
6. Inspect terminals at both ends of the circuit using the correct terminal tools and OEM views. Check for backed-out pins, spread terminals, corrosion, or overheated plastic. Pay attention to any sign of prior probing. Repair terminal fit issues with approved terminal repair parts, not generic crimp splices in SRS zones.
7. With the system still depowered, perform the OEM-approved circuit check for an “open” condition. Use Toyota’s specified SRS check harness or simulator tool if required by the procedure. Validate continuity of each conductor from the SRS ECU side to the knee airbag connector side. Also check for high resistance by gently loading the circuit per OEM direction. Do not measure inflator resistance directly.
8. Run a controlled wiggle test on the harness while monitoring the scan tool. Use a snapshot recording if your tool supports it. Move the harness at the knee bolster, steering column area, and any pass-through points. An intermittent open often appears when the harness flexes near a bracket or splice point.
9. If the circuit checks good end-to-end with approved methods, isolate the fault by dividing the circuit. Test from the SRS ECU connector to an intermediate connector, then from that point to the knee airbag connector. This approach finds hidden opens in sub-harnesses without guessing parts.
10. After you correct the root cause, restore the vehicle to a safe assembled state. Repower the SRS system per Toyota procedure. Clear SRS DTCs with the scan tool and rerun the health check. Confirm B1861 does not return on key-on and that the SRS warning lamp proves out normally.

Professional tip: Treat B1861 as a suspected trouble area, not a confirmed failed knee airbag. Toyota SRS logic often sets an “open” when the ECU loses the expected circuit signature. Connector fit and terminal tension cause many repeat comebacks. Fixing the pin fit and routing usually solves the problem without replacing the airbag module.

Possible Fixes

• Reseat and lock the knee airbag squib connector: Restore full terminal contact and confirm CPA locks hold under light pull.
• Repair terminal fit or replace damaged terminals using OEM-approved parts: Correct backed-out, spread, or corroded terminals that create an open circuit.
• Repair or replace the affected harness section: Fix broken conductors, chafing damage, or an open in a sub-harness found during circuit isolation.
• Correct harness routing and secure with proper retainers: Prevent repeat opens from harness strain against dash brackets or the steering column.
• Address SRS ECU connector issues: Clean, repair, or replace ECU-side terminals if testing proves an open at the ECU connection point.
• Replace the knee airbag module only after circuit verification: Replace the inflator module only when approved testing rules out wiring, connectors, and terminal issues.

Can I Still Drive With B1861?

You can usually drive a 2015 Toyota Auris with B1861, but you should treat the SRS as compromised. This code means the SRS ECU sees an open circuit in the driver knee airbag squib circuit. That knee airbag may not deploy in a crash. In some Toyota strategies, the SRS ECU may also disable related airbags to prevent unintended deployment. Drive only as needed to reach a qualified repair facility. Do not attempt DIY probing or bypassing. Follow Toyota SRS depowering procedures before any inspection, and use OEM-approved test methods only.

How Serious Is This Code?

B1861 is safety-critical, not a drivability complaint. The car will often run normally, so many owners treat it as an inconvenience. That thinking is wrong. An open squib circuit points to a high resistance or broken path in a deployment circuit. The SRS system relies on precise circuit integrity. Diagnosis requires SRS-certified technician training, Toyota service information, and a scan tool with full SRS access. Do not use standard test lights or back-probe methods on SRS connectors. Fix the root cause before you rely on the restraint system.

Common Misdiagnoses

Technicians often replace the knee airbag module first because the DTC “names” the component. That wastes money and can create new faults. Another common mistake involves probing the squib connector with a standard meter lead, which can spread terminals or damage shorting bars. Shops also miss connector issues at the SRS ECU or at the knee airbag because they skip a harness wiggle test and connector tension check. Many generic scan tools show limited SRS data, so the tech clears codes or guesses. Use Toyota-capable SRS diagnostics, confirm the open with approved methods, and verify harness integrity before parts.

Most Likely Fix

The most frequent confirmed repair directions for B1861 involve correcting an open circuit, not replacing the SRS ECU. Start with Toyota-approved inspection of the driver knee airbag squib connector and harness segment for terminal spread, poor seating, or damage. Next, verify continuity through the harness path per Toyota procedures, including connector lock engagement and pin fit. If tests confirm the airbag module side or harness side opens, repair the wiring or replace the affected sub-harness or connector. Replace the knee airbag module only after you prove the circuit and connectors pass and the fault follows the component.

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