Subaru B1572 on 2017-2019 Legacy: Immobilizer Circuit Fault Causes & Fixes

What's Unique About the 2017-2019 Subaru LEGACY

For this specific generation of Legacy and its platform-mate, the Outback, Subaru issued Technical Service Bulletin (TSB) 07-181-20 because the official factory service manual was missing a critical diagnostic step. 🎬 Watch: Diagnostic overview for the B1572 immobilizer circuit code. Technicians were often misdiagnosing the problem and replacing expensive parts unnecessarily. The TSB clarifies that before starting diagnostics for B1572, it is essential to check for any other codes stored in the immobilizer module itself and address those first, as they could be the root cause.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Engine will not start (cranks but does not fire).
• Intermittent stalling, sometimes immediately after starting.
• Flashing red security indicator light on the dashboard after a failed start attempt.
• Rough or unstable idle, even if the engine stays running.
• Remote start may not function correctly or at all.
• In some cases, other unrelated electrical issues may be present (windows, locks, lights) if the BIU is failing.
• Car may start after multiple attempts or after disconnecting and reconnecting the battery.

Most Likely Causes

1. Poor or Corroded Ground Connections 🔴 High Probability Multiple owner and technician reports point to corroded or loose engine-to-chassis ground straps as the root cause. A particularly problematic ground point is a multi-wire terminal located under the intake manifold, which can become loose and cause widespread communication issues. Other key grounds are located on either side of the engine bay, connecting the block to the chassis.
   How to confirm: Visually inspect all major ground straps. A key one is a braided strap near the passenger side strut tower. 🎬 See how to install a grounding kit to fix communication issues. The most critical one to check is the ground bolt under the intake manifold; it may feel loose to the touch. Use a multimeter to check for resistance between the engine block and the negative battery terminal; it should be near zero ohms. The factory manual specifies checking the ground circuit for the immobilizer module itself, which should be less than 10 Ω.
   Typical fix: Clean the contact points of the ground connections with a wire brush and ensure they are securely fastened. For the ground under the intake, tighten the bolt securing the bracket. If a strap is badly corroded or broken, it must be replaced.
   Est. part cost: $15-$50
2. Faulty Body Integrated Unit (BIU) 🟡 Medium Probability The BIU is the central computer for all body electronics, including the immobilizer. Internal circuit failure, sometimes due to moisture intrusion, voltage spikes, or age, can disrupt communication and trigger this code.
   How to confirm: This typically requires a professional scan tool (like the Subaru Select Monitor) to check for internal BIU faults and to monitor communication data. Diagnosis is often a process of elimination after confirming grounds and wiring are good.
   Typical fix: The BIU must be replaced. A new BIU must be professionally programmed by a dealer. 🎬 Watch: How to program a replacement Body Integrated Unit. A more cost-effective option is to use a mail-in 'cloning' service, where the immobilizer data from your original BIU is transferred to a compatible used unit, making it plug-and-play.
   Est. part cost: $250-$600 (New OEM), $50-$200 (Used) + Cloning Service
3. Wiring Harness or Connector Damage ⚪ Low Probability While less common, damage to the wiring or connectors between the BIU and the ECM can interrupt the communication signal required for engine start authorization. This can be caused by rodent damage, previous improper repairs, or chafing.
   How to confirm: A technician will perform a continuity and resistance test on the specific wires in the circuit as detailed in the factory service manual. This includes checking for opens or shorts between the BIU and ECM.
   Typical fix: Repair the damaged section of the wiring harness or replace the affected connector pigtail.
   Est. part cost: $20-$100

Diagnosis Steps

1. Connect a professional-grade OBD-II scanner capable of reading manufacturer-specific codes from all modules (Engine, Body, Immobilizer).
2. IMPORTANT (per TSB 07-181-20): Before diagnosing B1572, access the Immobilizer Control Module and check for any other stored DTCs. If any are found, diagnose and resolve them first.
3. If no other immobilizer codes are present, proceed with B1572 diagnosis.
4. Thoroughly inspect, remove, clean, and tighten all engine-to-chassis and transmission-to-chassis ground straps. Pay special attention to the braided straps on both sides of the engine bay and the main ground point under the intake manifold.
5. Check the main battery terminals and ground connection to the chassis for corrosion and tightness. Ensure the battery itself is healthy and providing stable voltage.
6. Inspect the wiring harness and connectors at the Body Integrated Unit (BIU), located under the driver's side dash near the fuse box, and at the Engine Control Module (ECM) for any visible signs of damage, corrosion, or loose pins.
7. If grounds and wiring appear to be in good condition, the fault is likely within the BIU itself. This step requires advanced diagnostics with a dealer-level tool to confirm.
8. If the BIU is determined to be faulty, it will need to be replaced and programmed, or the original unit can be sent with a donor for cloning.

Parts You'll Likely Need

• Engine Ground Strap — This is a frequently reported cause of the communication failure. Corrosion or a loose connection on these straps creates electrical noise and resistance, disrupting module communication.
  Trusted brands: Subaru (OEM), Dorman
  OEM price range: $25-$50
  Aftermarket price range: $15-$30
• Body Integrated Unit (BIU) (OEM #88281AL30B, 88281AL31B (Example Part Numbers - MUST match original)) — As the control center for the immobilizer, an internal failure of the BIU is the next most likely cause after grounds are ruled out. Part number must be matched exactly to the original unit for compatibility.
  Trusted brands: Subaru (OEM)
  OEM price range: $400-$600
  Aftermarket price range: $150-$350 (Used/Remanufactured)

Related Codes That Often Appear With This One

• P1603 — This code often logs an 'Engine Stall History'. It doesn't indicate a cause, but rather documents the stalling events that are a direct symptom of the B1572 immobilizer fault.
• U0140 — This is a generic code for 'Lost Communication With Body Control Module'. It can appear with B1572 if the Body Integrated Unit (BIU) is the source of the problem.
• U0131 — Lost Communication with Power Steering Control Module. This can be logged along with B1572 when a major ground fault disrupts communication across multiple modules on the CAN bus.

Technical Service Bulletins (TSBs) & Recalls

• 07-181-20: Provides a missing diagnostic step, warning technicians to check for other immobilizer codes first to prevent unnecessary parts replacement.

Platform-Specific Known Issues

• Subaru TSB 07-181-20 was issued specifically for 2017-2019 Legacy and Outback models to correct a missing step in the service manual's diagnostic procedure for DTC B1572, which could lead to misdiagnosis.
• A hidden ground connection under the intake manifold is a common failure point on this platform, causing intermittent stalling and a cluster of communication codes including B1572.

Mechanic-Grade Diagnostic Values

• Immobilizer Control Module (IMM ECM) ground circuit resistance — expected: Less than 10 Ω. Failure: A value of 10 Ω or higher indicates an open or high-resistance ground circuit.
• Immobilizer Control Module (IMM ECM) ignition circuit voltage — expected: 10 V or more. Failure: A value less than 10 V indicates an open or short in the harness between the IMM ECM and the ignition switch.

Scan Tool Commands That Help

• Subaru Select Monitor: System Operation Check Mode > Immobilizer System — This function performs an 'interface check' to verify the communication line between the ECM and the immobilizer module is not shorted or open.
• Subaru Select Monitor: Immobilizer Registration — Required after replacing the Body Integrated Unit (BIU) or keys to register the components with each other and allow the engine to start.
• Subaru Select Monitor: Read DTCs from Immobilizer Module — This is the first step mandated by TSB 07-181-20. Before diagnosing B1572 in the ECM, you must check the immobilizer module itself for any codes that could be the root cause.

Wiring & Ground Locations

• B141 — Connector for the Immobilizer Control Module (IMM ECM).. This is the connector where diagnostic tests for power and ground to the immobilizer module are performed. Specifically, terminals #2 and #3 are grounds, and terminal #12 is the ignition power supply.
• B280 — A connector for the Body Integrated Unit (BIU).. This is one of the main connectors for the BIU. Diagnostic charts reference checking continuity and for shorts to ground at pins on this connector when diagnosing related immobilizer codes.
• Intake Manifold Ground — Bolted to the top of the intake manifold.. The ECU gets its primary ground reference from the intake manifold. If this connection is poor (due to corrosion, looseness, or non-conductive spacers like TGV deletes), it can cause a host of communication errors, including B1572.
• Chassis Ground Points — Braided straps located on both sides of the engine bay, connecting the engine block to the chassis/unibody.. These are the main grounding paths between the engine and the rest of the vehicle's electrical system. Corrosion or looseness here is a very common cause of this code.

Real Owner Repair Stories

• YouTube channel 'Just B. Handy' (Subaru Outback (same platform as Legacy)) — Rough idle and remote start not working well. No check engine light was on, but a BlueDriver scanner found the stored B1572 code.
  ❌ Tried (didn't work) The owner did not report trying other fixes first, but took the car to the dealer armed with the TSB information.
  ✅ What actually fixed it The dealer, following TSB 07-181-20, performed diagnostics and found two corroded engine-to-chassis ground wires. Cleaning the connections on both sides of the engine resolved the code and the symptoms.
• Reddit user ElectroReactance in r/WRX (Subaru WRX (different model, but same code and similar electrical architecture)) — Intermittent no-start where the engine would fire for 2 seconds and then die, with the security light flashing.
  ❌ Tried (didn't work) Replacing all ground connections did not initially solve the problem.
  ✅ What actually fixed it The engine harness ground that connects to the fuel rail shield was tight, but the bolts holding the fuel rail shield itself to the engine were loose. Tightening the shield's mounting bolts restored the ground path and fixed the issue.

OEM Part Supersession History

• 82201AL30B → 82201AL30C — Logic update for Body Integrated Unit (BIU) operation.
  Heads up: Applies to 2018 Legacy/Outback with push-button start. The new part number requires a specific PAK file and decryption keyword for reprogramming.
• 82201AL31B → 82201AL31C — Logic update for Body Integrated Unit (BIU) operation.
  Heads up: Applies to 2018 Legacy/Outback with key start models. The new part number requires a specific PAK file and decryption keyword for reprogramming.