P2404 on 2017-2024 Subaru Impreza: EVAP Pump Circuit Causes and Fixes

What's Unique About the 2017-2024 Subaru IMPREZA

For this generation of Subaru Impreza and its platform mates, the P2404 code is very frequently a software issue rather than a hardware failure. Subaru has released multiple Technical Service Bulletins (TSBs) acknowledging that the ECM's logic can incorrectly flag this code, sometimes because the leak detection pump's motor temporarily seizes in a way the original software interprets as a fault. This makes checking the software version and performing a dealer reprogram the most critical first step, which is unique compared to other vehicles where this code often points directly to a failed pump.

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 illuminated.
• Disabling of safety systems like Subaru EyeSight (a common side effect of any Check Engine Light on modern Subarus).
• Disabling of Reverse Automatic Braking (RAB) system.
• Code may appear more frequently during cold weather or after a hard frost, according to owner reports.

Most Likely Causes

1. Outdated Engine Control Module (ECM) Software 🔴 High Probability Subaru has issued multiple TSBs (e.g., 09-118-24R for 2024 models, 09-122-24R for 2017-2023 models) for this generation, indicating the original ECM logic is too sensitive and can misinterpret signals from the leak detection pump, often due to the pump's motor temporarily seizing.
   How to confirm: A Subaru dealer or a shop with a Subaru Select Monitor (SSM) tool can check the current software calibration ID (CID) against the latest versions listed in the TSBs.
   Typical fix: Reprogramming the ECM with the updated software file provided by Subaru. This is the first step in the official diagnostic procedure.
   Est. part cost: $0 (if under warranty), $100-$200 (dealer labor for reprogram)
2. Faulty EVAP Leak Detection Pump (ELCM) 🟡 Medium Probability Forum discussions and dealer feedback confirm the pump itself is a very common failure item. Owners widely report the part being on backorder for weeks or months, suggesting a high failure rate and supply issue. The pump's internal motor or sensor can fail, leading to an out-of-range electrical signal.
   How to confirm: After confirming the ECM software is up-to-date, a technician should follow the TSB diagnostic flow: perform an "Evaporative System Leak Test" using the SSM tool. A failing result (vacuum value not lower than -0.26 PSI) points to a faulty pump.
   Typical fix: Replace the EVAP Leak Detection Pump, also called the Evaporative Leak Check Module (ELCM).
   Est. part cost: $150-$300
3. Wiring or Connector Issue ⚪ Low Probability
   How to confirm: Visually inspect the wiring harness and connector leading to the leak detection pump (located at the rear of the vehicle, near the charcoal canister) for any signs 🎬 Watch: Step-by-step instructions for accessing the canister and pump area. of corrosion, rodent damage, or loose pins. Use a multimeter to check for continuity and proper voltage from the ECM. A resistance value of 1 Ohm or higher can indicate a damaged harness.
   Typical fix: Repair the damaged section of the wiring harness or clean/replace the connector.
   Est. part cost: $5-$50

Rare But Worth Checking

• Faulty Engine Control Module (ECM): This is very rare. The ECM should only be considered as the cause after the software has been updated and both the pump and its wiring have been tested and confirmed to be good.

Diagnosis Steps

1. Scan the vehicle for trouble codes to confirm P2404 is present.
2. Check with a Subaru dealer or consult TSBs to determine if the vehicle's ECM software is outdated. TSB 09-122-24R applies to 2017-2023 models and TSB 09-118-24R applies to 2024 models.
3. If a software update is available, have the ECM reprogrammed. This is the most likely fix and should be the first step.
4. If the code returns after the update, locate the EVAP leak detection pump (ELCM). It is at the rear of the vehicle, typically mounted above the rear subframe/differential area, 🎬 See this walkthrough on how to remove the leak detection pump. behind the fuel tank.
5. Visually inspect the pump, its electrical connector, and harness for any obvious damage, corrosion, or loose connections.
6. Per TSB 09-118-24R, inspect the ELCM drain tube for signs of water or oil ingress.
7. Using a Subaru Select Monitor (SSM), perform the "Evaporative System Leak Test".
8. If the test fails (the TSB specifies a vacuum value should be lower than -0.26 PSI), the ELCM pump is faulty and must be replaced.
9. If the pump is replaced, the TSB may require reprogramming the ECM as the final step.
10. If the pump and wiring test good and the software is current, the issue may be a more complex, intermittent problem or a rare ECM hardware fault.

Parts You'll Likely Need

• EVAP Leak Detection Pump / Evaporative Leak Check Module (ELCM) (OEM #42084FL17C) — This is the primary hardware component that fails when the issue is not software-related. Owner and dealer reports confirm it's a high-failure part, often on backorder. Note: Part numbers may vary by model year and manufacturing location (US vs. Japan); always confirm with a dealer using your VIN.
  Trusted brands: Subaru (OEM)
  OEM price range: $200-$300
  Aftermarket price range: $150-$250

Related Codes That Often Appear With This One

• P0087 — Subaru TSB 09-118-24R, which applies to 2024 Impreza models, mentions that the same ECM software update that fixes P2404 also includes enhanced logic to stabilize fuel pressure, preventing P0087 (Fuel Rail/System Pressure - Too Low).

Technical Service Bulletins (TSBs) & Recalls

• 09-118-24R: Announces new ECM reprogramming files for 2024 Impreza & Crosstrek to address DTC P2404 detection, caused by the internal pump for the Evaporative Leak Check Module (ELCM) temporarily seizing. Also addresses P0087.
• 09-122-24R: Provides reprogramming files for DTC P2404 on 2017-2023 Impreza, 2018-23 Crosstrek, 2019-23 Forester, and 2020-22 Legacy/Outback, citing the same cause of the ELCM pump temporarily seizing.

Platform-Specific Known Issues

• A known software issue addressed by Subaru TSBs #09-118-24R and #09-122-24R causes the ECM to incorrectly detect a fault with the leak detection pump circuit.
• Owners on forums consistently report that the leak detection pump is a common failure item and has been on significant backorder, indicating a widespread issue.
• The Check Engine Light for P2404 will disable the EyeSight and Reverse Automatic Braking (RAB) systems as a default safety measure.

Mechanic-Grade Diagnostic Values

• ELCM Vacuum Value during 'Evaporative System Leak Test' — expected: Value should be lower than -0.26 PSI.. Failure: A value of -0.26 PSI or higher indicates a fault in the ELCM system, likely the pump itself.
• Reprogramming Power Supply Voltage — expected: A steady 13.5 Volts.. Failure: Voltage rising above 14 Volts can cause the reprogramming procedure to abort.
• Wiring Harness Continuity Check — expected: Resistance should be less than 1 Ohm.. Failure: A resistance value of 1 Ohm or higher indicates a damaged or corroded section of the harness that needs repair.

Hidden / Shadow Codes Worth Checking

• MID: $3C, TID: $C4, Scaling ID: $FE: These are not DTCs but specific Monitor and Test IDs within the Subaru Select Monitor (SSM) software. They correspond to the vacuum value result from the 'Evaporative System Leak Test' used to confirm a faulty ELCM pump. (see via Must be viewed using the Subaru Select Monitor (SSM) tool while performing the guided leak test function described in Subaru TSBs.)

Scan Tool Commands That Help

• Subaru Select Monitor (SSM): Evaporative System Leak Test — This is the primary diagnostic function to run after confirming the ECM software is up-to-date. It commands the ELCM pump to run and measures the vacuum it creates to determine if the pump is functioning correctly.
• Subaru Select Monitor (SSM): ELCM Pump Active Test — This function is used when an initial leak test is inconclusive. It runs the pump continuously for an extended period (TSB suggests 35+ minutes) to try and induce an intermittent failure (like a temporary seizure) that can then be caught by re-running the 'Evaporative System Leak Test'.

Wiring & Ground Locations

• ELCM Connector — The Evaporative Leak Check Module (ELCM) is located at the rear of the vehicle, mounted near or on the charcoal canister assembly, typically above the rear subframe.. This is the main electrical connector for the pump. Technicians can back-probe pins 3 (Red wire) and 4 (Gray wire) to monitor the pump's voltage signal and ground control from the ECM with an oscilloscope to diagnose circuit performance issues.

Real Owner Repair Stories

• Reddit user AcademicViolinist355 in r/WRX (2018 Subaru WRX) — Check Engine Light came on the morning after a dealership oil change.
  ❌ Tried (didn't work) Tightening the gas cap.
  ✅ What actually fixed it The problem was fixed by replacing the 'Evap Emissions System Leak Detection' assembly, part number 42084VA030. The user admitted their habit of topping off the fuel tank after the nozzle clicks off was the likely cause of the part failure.

"I Checked Everything" — The Actual Cause

• A standard EVAP smoke test will typically pass on a vehicle with only a P2404 code. This is because the code indicates an electrical performance issue with the pump's sense circuit, not a physical leak in the EVAP hoses or canister. Technicians may waste time looking for a leak when the root cause is either faulty ECM software logic or an internal failure of the leak detection pump motor/sensor itself.

OEM Part Supersession History

• 42084VA030 → 42084VA031 — Unknown, but supersession often indicates a design or material improvement to address a known failure pattern.
  Heads up: This part is for a WRX, but demonstrates that Subaru has revised these components. The Impreza-specific part (42084FL17C) may also have supersessions; always verify with a dealer using the VIN.

Model Year Variations Within This Range

• 2017-2023 Impreza: The software fix for P2404 is covered by TSB #09-122-24R.
• 2024 Impreza: The software fix for P2404 is covered by TSB #09-118-24R, which also includes a logic update to address DTC P0087 (Fuel Rail/System Pressure - Too Low).
• All (2017-2024): According to a technician on a forum, Japan-built and US-built Imprezas may use different, non-interchangeable part numbers for the leak detection pump. It is critical to confirm the correct part using the vehicle's VIN.