B1816 – P squib (dual stage – 2nd step) circuit open (Lexus)

B1816 means the Lexus ES has a fault in the airbag system that can disable part of the front passenger airbag deployment. You will usually see the SRS/airbag warning light, and the system may not protect occupants as designed in a crash. This is a Lexus manufacturer-specific code, and the exact monitoring logic can vary by platform. According to Lexus factory diagnostic data, this code indicates an open circuit in the P squib (dual stage, 2nd step) circuit. Because this circuit belongs to the SRS, follow Lexus depowering procedures before touching connectors. Use only OEM-approved SRS test methods.

B1816 Quick Answer

B1816 on a Lexus ES points to an open circuit in the front passenger airbag inflator “P squib” second-stage circuit. Diagnose the wiring and connectors first, using SRS-safe procedures and a scan tool with full SRS access.

What Does B1816 Mean?

Official definition: “P squib (dual stage – 2nd step) circuit open.” In plain terms, the SRS ECU sees an incomplete electrical path to the passenger airbag inflator’s second-stage igniter. In practice, the airbag warning light turns on and the system may inhibit that stage, or the entire passenger module, depending on Lexus platform logic.

What the module checks: the SRS ECU monitors the squib circuit for continuity and expected electrical characteristics through the harness, connectors, and the airbag module igniter. Why it matters: an “open” does not prove a bad airbag. It only identifies a suspected trouble area. You must confirm the open using Lexus-approved SRS circuit checks, because incorrect probing can deploy an airbag or damage the SRS ECU.

Theory of Operation

Under normal operation, the Lexus SRS ECU continuously supervises each squib circuit. A dual-stage inflator uses two separate igniters. The ECU can command stage 1, stage 2, or both, based on crash severity and timing.

For B1816, the ECU decides the “P squib 2nd step” circuit looks open. That happens when the circuit path breaks anywhere between the ECU and the inflator igniter. High resistance at a connector can also mimic an open during supervision. Physical damage near the seat, dash, or SRS harness routing often causes this code on an ES.

Symptoms

You will typically notice one or more of these symptoms with B1816 on a Lexus ES:

• SRS warning Airbag/SRS warning light stays on
• Message “Check SRS Airbag System” or similar warning appears
• Passenger protection Passenger airbag function may be limited or disabled
• Stored DTC B1816 stores as current or history in the SRS ECU
• Readiness SRS system status shows malfunction on an SRS-capable scan tool
• Intermittent fault Light may change with temperature, vibration, or seat movement
• Crash data caution Post-collision repairs may coincide with this code

Common Causes

• Open in the 2nd-stage squib circuit wiring: A broken conductor in either squib leg prevents the SRS ECU from seeing the expected loop integrity.
• Loose or unlatched SRS connector at the airbag module: Incomplete terminal engagement creates an intermittent or hard open that sets immediately at key-on.
• Terminal fretting or corrosion at the squib connector: Micromovement and oxidation increase resistance until the ECU interprets the circuit as open.
• Harness damage in the instrument panel area: Pinched, rubbed-through, or previously repaired wiring near brackets or sharp edges can open the second-stage pair.
• Clock spring/spiral cable open (if the affected squib routes through it): A cracked ribbon conductor can open one stage while the other stage still tests normal.
• Improper probing or prior test damage to SRS terminals: Standard meter leads can spread terminals or deform shorting bar features, creating a new open.
• Connector shorting bar not returning to the correct position: A mispositioned internal shorting mechanism can interrupt the ECU’s circuit path and mimic an open.
• Incorrect or incompatible airbag component installed: A mismatched dual-stage module or sub-harness can present a different circuit configuration that the ECU reads as open.

Diagnosis Steps

Use a scan tool with full Lexus SRS access, OEM-level wiring information, and OEM-approved SRS test adapters. Keep a DMM available for voltage-drop checks on SRS ECU powers and grounds, not for probing squib pins. Follow Lexus depowering procedures before touching any SRS connector. Do not use test lights or jumper wires on SRS circuits.

1. Confirm DTC B1816 in the SRS/Airbag menu and record freeze frame data. Focus on battery voltage, ignition state, and any companion SRS DTCs that indicate a shared connector or power feed. Note whether the code shows as pending or confirmed/stored, since a hard open often returns immediately at key-on. Freeze frame captures the conditions when the fault set, while a scan tool snapshot helps you catch an intermittent during harness manipulation.
2. Perform a fast visual inspection of the suspected circuit path before any meter work. Look for recent dash, steering wheel, seat, or collision repairs on the Lexus ES that could disturb SRS connectors. Check harness routing for pinch points, abrasion, or aftermarket fasteners. Verify that all yellow SRS connectors appear fully seated and locked.
3. Check SRS-related fuses and power distribution feeds first, using the correct fuse ID from service information. Verify that the fuse blades grip tightly and show no heat discoloration. A power feed issue can trigger multiple SRS codes and can mislead you into chasing a squib circuit open.
4. Verify SRS ECU power and ground integrity with voltage-drop testing under load. Backprobe only at approved test points, not at squib terminals, and keep the circuit operating during the test. Target less than 0.1 V drop on grounds with the system powered. Do not rely on continuity checks alone because high resistance often hides with no load.
5. Cycle the ignition as directed by service information and re-check whether B1816 returns immediately. If the code resets at key-on every time, treat it as a hard fault and stay focused on connector seating, terminal tension, and a true open circuit. If it returns intermittently, plan for wiggle testing and scan tool snapshot capture.
6. Depower the SRS system using Lexus procedures and wait the specified time before disconnecting any SRS connector. Confirm the vehicle remains depowered during the inspection. Do not probe squib connectors with standard meter leads, and do not use resistance measurements across an airbag inflator unless the OEM procedure explicitly allows it with an approved tool.
7. Inspect the connectors in the suspected “P squib (dual stage – 2nd step)” circuit. Check CPA locks, terminal push-out, bent pins, water intrusion signs, and damaged connector housings. Pay close attention to the shorting bar and terminal fit because minor deformation can create an open that looks “random” on a scan tool.
8. Inspect the harness segment for the 2nd-stage pair from the SRS ECU toward the airbag module. Look for chafing at brackets, tight bends, crushed conduit, and prior splices. If service information shows the circuit routes through a spiral cable or intermediate connector, inspect those interfaces with the same care.
9. Use OEM-approved SRS test adapters and the service manual’s circuit check method to verify circuit continuity and isolation without deploying an inflator. Do not substitute homemade resistors or jumper leads. Confirm both legs of the 2nd-stage circuit show the expected path and no opens at intermediate connectors.
10. If the fault appears intermittent, run a controlled wiggle test while monitoring SRS data and DTC status on the scan tool. Trigger a scan tool snapshot to capture live data at the moment the code changes state. Concentrate movement near the connectors you touched, harness clamps, and known flex points, rather than randomly shaking the loom.
11. After repair, reassemble connectors with locks fully engaged and restore harness routing and strain relief. Repower the SRS system per Lexus procedure, then clear SRS DTCs with a capable scan tool. Confirm B1816 stays cleared through multiple key cycles and a functional self-check, and verify no new SRS codes appear.

Professional tip: Treat B1816 as a suspected trouble area, not a confirmed failed airbag. On Lexus dual-stage circuits, the most repeatable failures come from connector issues after interior work. Use the correct SRS connector handling steps every time. One spread terminal can create a persistent “circuit open” that survives parts swapping.

Possible Fixes

• Reseat and lock the affected SRS connectors: Correct connector engagement and CPA lock position when inspection shows incomplete seating or loose retention.
• Repair the open in the 2nd-stage squib harness: Restore conductor integrity and strain relief using OEM-approved wiring repair methods for SRS circuits.
• Replace a damaged connector housing or terminals: Correct terminal tension, push-out, or shorting bar damage found during close inspection.
• Replace the spiral cable/clock spring if testing proves an open in that path: Install only after you verify the circuit routes through it and the open repeats with movement.
• Correct an incompatible or incorrect airbag-related component installation: Verify part numbers and configuration match Lexus ES application before condemning wiring or the ECU.
• Restore SRS ECU power/ground integrity: Clean and secure power distribution or ground points if voltage-drop testing shows excessive loss under load.

Can I Still Drive With B1816?

You can usually drive a Lexus ES with B1816 because it does not affect engine power or braking. Do not treat it as “safe,” though. This code points to an SRS squib circuit open on the passenger airbag dual-stage second step. That means the Supplemental Restraint System may not deploy as designed in a crash. The SRS warning light often stays on, and the system can disable part of the airbag strategy. Avoid carrying passengers until you fix it. Do not disconnect SRS connectors or move seat/trim components as a quick check. Follow Lexus depower procedures first, and use a scan tool that fully supports SRS diagnostics.

How Serious Is This Code?

B1816 is serious because it involves an airbag igniter circuit. You may not notice any drivability change, so it can feel like an inconvenience. In reality, the risk sits in a collision event. Lexus uses dual-stage deployment logic on many platforms, and the “2nd step” circuit supports higher output when needed. An open circuit can prevent that stage from firing, or it can force the SRS ECU to inhibit deployment for safety. Treat the SRS system as potentially compromised until you complete diagnosis. This work requires SRS-safe handling, OEM-approved test methods, and the correct equipment. If you lack SRS training, do not DIY this repair.

Common Misdiagnoses

Technicians often replace the passenger airbag module or spiral cable without proving an open circuit at the correct connector. Another common error involves probing squib circuits with a standard DMM lead set or test light. That can damage terminals or create an unsafe condition. Shops also miss connector problems caused by prior dash work, water intrusion, or a partially latched SRS connector with a backed-out terminal. Some people chase battery voltage issues and ignore the SRS ECU’s internal continuity check logic. Avoid wasted parts by verifying the exact squib pair, checking for high resistance at connector interfaces, and confirming harness integrity end-to-end using OEM-approved SRS test adapters.

Most Likely Fix

The most common confirmed repairs for B1816 involve restoring continuity in the passenger airbag “P squib” second-stage circuit. That usually means correcting a poor connection, terminal fretting, corrosion, or a damaged harness segment between the SRS ECU and the passenger airbag connector. Do not assume the airbag module failed. First prove the open with the correct Lexus procedure and approved shorting bars or service tools. After repair, run an SRS health check with a capable scan tool and confirm the code stays cleared after key cycles and a verification drive under the enable criteria Lexus specifies for SRS self-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: April 3, 2026