P0087 on 2020-2024 Subaru Legacy: Low Fuel Rail Pressure Causes & Fixes

What's Unique About the 2020-2024 Subaru LEGACY

For this specific generation of the Subaru Legacy, which features direct injection on both the standard 2.5L and turbo 2.4L engines, P0087 is a well-documented issue. Subaru has officially acknowledged the problem by releasing multiple Technical Service Bulletins (TSBs). These bulletins provide fixes for both software and hardware, pointing to flawed ECM logic that can be fixed with a reprogram, and a high-pressure fuel pump that was prone to failure. TSB #09-123-24 specifies the pump failure was due to engine vibrations affecting an internal discharge valve, a problem addressed in a redesigned part.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on
• Reduced engine power or sluggish acceleration
• Engine hesitation or stumbling, especially under load
• Rough idle or engine running rough
• Engine stalling, especially under load
• Difficulty starting the engine or extended cranking
• Decreased fuel economy
• Smell of fuel on startup

Most Likely Causes

1. Outdated Engine Control Module (ECM) Software 🔴 High Probability → Shop Engine Control Module (ECM) Subaru issued TSBs #09-120-24 and #09-120-24R specifically to update the ECM logic. The new software is designed to "optimize stabilization of fuel pressure" and correct faulty self-diagnosis routines.
   How to confirm: A Subaru dealer or a shop with advanced scan tools can check the current software calibration ID against the latest version specified in the TSBs.
   Typical fix: Reprogramming the ECM with the updated software file provided by Subaru. This is the least invasive and often the first step recommended by the manufacturer.
   Est. part cost: $0
2. Failing High-Pressure Fuel Pump (HPFP) 🔴 High Probability → Shop Fuel Pump Subaru issued TSB #09-123-24 announcing a redesigned HPFP. It states the original pump's internal discharge valve performance could be affected by engine vibrations, causing unstable fuel pressure.
   How to confirm: Use a scan tool to monitor 'Actual Fuel Rail Pressure' vs. 'Desired Fuel Rail Pressure' live data. If the actual pressure drops significantly below the desired pressure, especially under acceleration, the HPFP is a likely culprit. A technician may also inspect the part number on the pump to see if it is the older, pre-redesign version.
   Typical fix: Replace the high-pressure fuel pump with the new, redesigned part. The OEM part number for the redesigned pump on the 2.4L Turbo is 16625AA110.
   Est. part cost: $600-$900
3. Failing Low-Pressure Fuel Pump (In-Tank) 🟡 Medium Probability → Shop Fuel Pump A safety recall (WRG-21R / NHTSA ID: 21V587) was issued for some 2020 Legacy models. The low-pressure pump impeller was manufactured with a lower density, which could cause it to deform and become inoperative, starving the HPFP of fuel. A warranty extension was also issued for this part (TSB #09-129-24).
   How to confirm: Check the vehicle's VIN against the recall database on Subaru's website or the NHTSA site. A technician can also perform a low-pressure fuel system test to see if pressure from the tank is below specification.
   Typical fix: Replace the in-tank low-pressure fuel pump assembly, which is covered by the dealer if the vehicle is part of the recall or warranty extension.
   Est. part cost: $0 (if under recall), $200-$400 otherwise
4. Clogged Fuel Filter 🟡 Medium Probability → Shop Fuel Filter
   How to confirm: First, test the pressure from the low-pressure fuel pump in the tank. If the pressure is below specification, the filter (which is often an integrated part of the pump assembly in modern cars) may be clogged.
   Typical fix: Replace the in-tank fuel filter or the entire low-pressure fuel pump assembly.
   Est. part cost: $350-$900
5. Faulty Fuel Rail Pressure Sensor ⚪ Low Probability → Shop Fuel Pressure Sensor
   How to confirm: Compare the scan tool's pressure reading to a mechanical gauge (if a port is available). If the readings differ significantly, or if the reading is erratic or stuck at a low value regardless of engine load, the sensor is likely faulty.
   Typical fix: Replace the fuel rail pressure sensor.
   Est. part cost: $100-$200

Rare But Worth Checking

• Leaking Fuel Injector(s): → Shop Fuel Injector A fuel injector stuck open can cause a drop in rail pressure, but would typically be accompanied by other codes like misfires (P030x) or a rich condition.
• Damaged wiring to the fuel pump or pressure sensor: → Shop Fuel Pump Frayed wires or a corroded connector can cause intermittent or incorrect signals, leading to the code.

Diagnosis Steps

1. Check for applicable Technical Service Bulletins (TSBs) and Recalls for your vehicle's VIN. The first step should be to see if the ECM reprogram TSBs or the low-pressure pump recall (WRG-21R) apply.
2. Connect an OBD-II scanner and check for any other stored trouble codes, paying special attention to camshaft correlation codes or P0088.
3. Using the scanner's live data function, monitor both Desired and Actual Fuel Rail Pressure. Note any discrepancies, especially during a test drive under acceleration.
4. If pressure is low, test the low-pressure fuel system first. Ensure the in-tank pump is delivering adequate pressure to the engine bay.
5. If an ECM update is available, have the dealer perform the reprogram as specified in the TSB. This is often the least expensive potential fix.
6. Inspect the part number on the high-pressure fuel pump. TSB #09-123-24 advises verifying the new part number is not already installed before proceeding with a replacement.
7. If the software is up-to-date, the low-pressure system is healthy, and pressure remains low under load, the high-pressure fuel pump is the most likely failed component and should be replaced.

Parts You'll Likely Need

• High-Pressure Fuel Pump (OEM #16625AA110 (for 2.4L Turbo engine)) — This part is the subject of a specific TSB (#09-123-24) where Subaru announced a redesign to fix the P0087 code, indicating a known hardware weakness due to engine vibrations.
  Trusted brands: Subaru (OEM)
  OEM price range: $600-$900
  Aftermarket price range: $450-$700
• Engine Control Module (ECM) Reprogramming — Subaru issued TSBs (#09-120-24, #09-120-24R) with updated software to correct diagnostic logic that could falsely trigger P0087. It's the official first step in diagnosis.
  OEM price range: $100-$200
  Aftermarket price range: N/A
• Low-Pressure Fuel Pump Assembly — Subject of safety recall WRG-21R for some 2020 models due to a defective impeller that can cause the pump to fail, leading to low system pressure.
  Trusted brands: Subaru (OEM)
  OEM price range: $200-$400
  Aftermarket price range: $150-$300

Related Codes That Often Appear With This One

• P0016, P0017, P0018, P0019 — Subaru TSBs #09-120-24 and #09-120-24R list these camshaft/crankshaft correlation codes alongside P0087 as being resolved by the same ECM reprogram. This suggests the original software had logic flaws affecting multiple diagnostic monitors.
• P0088 — Code P0088 is for "Fuel Rail/System Pressure - Too High". Seeing both P0087 and P0088 can indicate an erratic fuel pressure sensor or a major software logic issue.

Technical Service Bulletins (TSBs) & Recalls

• 09-123-24: Announces a design change to the high-pressure fuel pump to prevent DTC P0087.
• 09-120-24R: Provides new ECM reprogramming files to remedy DTC P0087 and others.
• 09-120-24: Announces availability of new reprogramming files for the Engine Control Module (ECM) developed to remedy DTC P0087.

Platform-Specific Known Issues

• Subaru has issued TSB #09-123-24 for a redesigned high-pressure fuel pump to specifically address code P0087, citing issues with the internal discharge valve and engine vibrations.
• Subaru has also released TSBs #09-120-24 and #09-120-24R, which provide new ECM software to fix faulty logic that causes P0087 to appear.
• A safety recall, WRG-21R (NHTSA ID: 21V587), was issued for the in-tank low-pressure fuel pump on some 2020 models, which can fail and cause a P0087 code.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System Pressure — expected: ~50 psi (340 kPa). Failure: Pressure significantly below 50 psi indicates a failing in-tank pump or clogged filter.
• High-Pressure Fuel System Pressure (at idle) — expected: 580 - 870 psi (4 - 6 MPa). Failure: Pressure that is unstable or drops below 580 psi at idle suggests an issue with the HPFP or its control.
• High-Pressure Fuel System Pressure (under load) — expected: Should rise significantly, often exceeding 2,000 psi (13.8 MPa).. Failure: Failure of actual pressure to match desired pressure on a scan tool during acceleration points to a weak HPFP or insufficient supply from the low-pressure pump.
• Fuel Rail Pressure Sensor Voltage (Key On, Engine Off) — expected: ~0.5V. Failure: Voltage stuck at 0V or a high value (e.g., 5V) indicates a sensor or wiring fault.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM): Fuel Pump Relay Test — This bidirectional command allows a technician to manually activate the low-pressure in-tank fuel pump to test its operation and measure its output pressure without the engine running. This is a critical first step to isolate a problem between the low and high-pressure systems.
• Subaru Select Monitor (SSM): Reset Fuel Learning Value — After replacing a major fuel system component like the high-pressure fuel pump or injectors, this function should be used to clear the ECM's learned fuel trim adjustments, allowing it to adapt to the new components correctly.

Wiring & Ground Locations

• Fuel Pump Assembly Connector — Under the rear seat, on top of the fuel tank access panel.. This is the primary connection for the in-tank low-pressure fuel pump. Corrosion or a loose connection here can cause the pump to fail intermittently or completely, starving the high-pressure pump of fuel and triggering P0087.
• High-Pressure Fuel Pump Connector — On the high-pressure fuel pump, mounted on the engine.. The wiring harness near this connector can be susceptible to chafing from engine vibrations. An intermittent short-to-ground on the signal wire can cause erratic pressure readings and trigger P0087, even when the pump itself is healthy.
• Fuel Rail Pressure Sensor Connector — On the end of the high-pressure fuel rail.. This 3-pin connector provides the ECM with the pressure reading. A voltage test here (Pin 1: 5V Ref, Pin 2: Ground, Pin 3: Signal) can quickly diagnose a faulty sensor versus a wiring issue.

Real Owner Repair Stories

• Reddit user on r/SubaruAscent (2019 Subaru Ascent (with same 2.4L FA24F engine) at 70,000 miles) — Car would not start after being driven for an hour in hot weather. When it did start, it would misfire, run terribly, and shut off. Code P0087 was stored.
  ❌ Tried (didn't work) Dealer could not replicate the issue initially.
  ✅ What actually fixed it The dealer eventually found that the in-tank low-pressure fuel pump was not properly supplying fuel to the high-pressure side and replaced it. This resolved the issue, highlighting that the low-pressure pump can fail even if a vehicle's VIN is not part of the official recall.
• Subaru Outback forum user (2020 Subaru Outback XT (with 2.4L FA24F engine)) — Check Engine Light with code P0087 and hesitation under acceleration.
  ❌ Tried (didn't work) N/A - Diagnosis was direct.
  ✅ What actually fixed it The dealer identified TSB 09-123-24, confirmed the vehicle had the older-style high-pressure fuel pump, and replaced it with the redesigned part. This was the final and successful fix.

When the Usual Fixes Don't Work

• In at least one documented case for the same FA24F engine, both an ECM reprogram and a high-pressure fuel pump replacement FAILED to fix the P0087 code. The ultimate cause was a failing in-tank low-pressure fuel pump, even though the vehicle was not part of the official recall. This underscores the importance of testing the low-pressure system's output before replacing the expensive high-pressure pump.

OEM Part Supersession History

• 16625AA100 (Likely) → 16625AA110 — Redesigned to optimize the internal discharge valve's performance and prevent unstable fuel pressure caused by engine vibrations, as per TSB 09-123-24.

Model Year Variations Within This Range

• 2020-2023 (and early 2024): These model years were likely built with the original high-pressure fuel pump design that is prone to failure from vibration. TSB #09-123-24 indicates the redesigned pump was incorporated into Legacy production starting with VIN R3007808, so vehicles built before that point are suspect.
• 2020-2022: These model years are the specific targets for the ECM reprogramming TSBs (#09-120-24, #09-120-24R). Later models (2023+) may have shipped from the factory with the updated software logic already installed.