The pump gets power through fuse 25 after the main relay. This fuse also controls the injectors. So that's NOT the problem. There could be a loose connection at the LDP or valve or broken wire.

DTC P2404 EVAP SYSTEM LEAK DETECTION PUMP SENSE CIRCUIT RANGE/PERFORMANCE

DTC detecting condition:
Detected when two consecutive driving cycles with fault occur.

Caution:

After servicing or replacing faulty parts, perform Clear Memory Mode See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Reading and Clearing Diagnostic Trouble Codes\Engine (H4DO) - Clear Memory Mode - Operation Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Clear Memory Mode>OPERATION., and inspection mode See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Verification Tests\Engine - Inspection Mode (H4DO) - Procedure Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Inspection Mode>PROCEDURE.

Use the check board when measuring the ECM terminal voltage and resistance.
Wiring diagram:
Engine Electrical System See: Diagrams\Electrical\Computers and Control Systems\System Diagram\Engine Electrical System - Wiring System Diagram Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM.

1. CHECK FOR ANY OTHER DTC ON DISPLAY.
Is any other DTC displayed?
Yes
Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". See: Diagnostic Trouble Code Descriptions\List of Diagnostic Trouble Code (DTC)\Engine (H4DO) Ref. to ENGINE (DIAGNOSTICS)(H4DO)>List of Diagnostic Trouble Code (DTC).
No
Go to 2.

2. CHECK LEAK CHECK VALVE ASSEMBLY PUMP.
Using the Subaru Select Monitor, operate the leak check valve assembly pump.

Note:

For detailed operation procedures, refer to the "Active Test". See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Scan Tool Testing and Procedures\Engine (H4DO) - Active Test - Operation Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Active Test.
Does the leak check valve assembly pump operate?
Yes
Go to 6.
No
Go to 3.

3. CHECK POWER SUPPLY OF LEAK CHECK VALVE ASSEMBLY.

1. Turn the ignition switch to OFF.
2. Disconnect the connector from the leak check valve assembly.
3. Turn the ignition switch to ON.
4. Measure the voltage between the leak check valve assembly connector and chassis ground. Connector & terminal

(R400) No. 3 (+) - Chassis ground (-):
Is the voltage 10 V or more?
Yes
Go to 4.
No

Repair the harness and connector.

Note:

In this case, repair the following item:

Open circuit in harness between the main relay and the leak check valve assembly connector

Poor contact of main relay connector

Poor contact of coupling connector

4. CHECK HARNESS BETWEEN ECM AND LEAK CHECK VALVE ASSEMBLY CONNECTOR.

1. Turn the ignition switch to OFF.
2. Disconnect the connector from ECM.
3. Measure the resistance of harness between ECM connector and the leak check valve assembly connector. Connector & terminal

(B134) No. 9 - (R400) No. 4:
Is the resistance less than 1 ohms?
Yes
Go to 5.
No

Repair the harness and connector.

Note:

In this case, repair the following item:

Open circuit in harness between ECM connector and the leak check valve assembly connector

Poor contact of coupling connector

5. CHECK HARNESS BETWEEN ECM AND LEAK CHECK VALVE ASSEMBLY CONNECTOR.

1. Measure the resistance of harness between ECM connector and chassis ground. Connector & terminal

(B134) No. 9 - Chassis ground:
Is the resistance 1 Mohms or more?
Yes
Replace the leak check valve assembly. See: Powertrain Management\Emission Control Systems\Leak Detection Valve\Specifications\Leak Check Valve Assembly (H4DO) - Specification Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES)(H4DO)>Leak Check Valve Assembly.
No
Repair the short circuit to ground in harness between ECM connector and leak check valve assembly connector.

6. CHECK LEAK CHECK VALVE ASSEMBLY SWITCHING VALVE.
Using the Subaru Select Monitor, operate the leak check valve assembly switching valve.

Note:

For detailed operation procedures, refer to the "Active Test". See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Scan Tool Testing and Procedures\Engine (H4DO) - Active Test - Operation Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Active Test.
Does the leak check valve assembly switching valve operate?
Yes
Go to 10.
No
Go to 7.

7. CHECK POWER SUPPLY OF LEAK CHECK VALVE ASSEMBLY.

1. Turn the ignition switch to OFF.
2. Disconnect the connector from the leak check valve assembly.
3. Turn the ignition switch to ON.
4. Measure the voltage between the leak check valve assembly connector and chassis ground. Connector & terminal

(R400) No. 5 (+) - Chassis ground (-):
Is the voltage 10 V or more?
Yes
Go to 8.
No

Repair the harness and connector.

Note:

In this case, repair the following item:

Open circuit in harness between the main relay and the leak check valve assembly connector

Poor contact of main relay connector

Poor contact of coupling connector

8. CHECK HARNESS BETWEEN ECM AND LEAK CHECK VALVE ASSEMBLY CONNECTOR.

1. Turn the ignition switch to OFF.
2. Disconnect the connector from ECM.
3. Measure the resistance of harness between ECM connector and the leak check valve assembly connector. Connector & terminal

(B134) No. 61 - (R400) No. 1:
Is the resistance less than 1 ohms?
Yes
Go to 9.
No

Repair the harness and connector.

Note:

In this case, repair the following item:

Open circuit in harness between ECM connector and the leak check valve assembly connector

Poor contact of coupling connector

9. CHECK HARNESS BETWEEN ECM AND LEAK CHECK VALVE ASSEMBLY CONNECTOR.

1. Measure the resistance of harness between ECM connector and chassis ground. Connector & terminal

(B134) No. 61 - Chassis ground:
Is the resistance 1 Mohms or more?
Yes
Replace the leak check valve assembly. See: Powertrain Management\Emission Control Systems\Leak Detection Valve\Specifications\Leak Check Valve Assembly (H4DO) - Specification Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES)(H4DO)>Leak Check Valve Assembly.
No
Repair the short circuit to ground in harness between ECM connector and leak check valve assembly connector.

10. CHECK EVAPORATIVE EMISSION CONTROL SYSTEM.
Perform drive cycle I. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Monitors, Trips, Drive Cycles and Readiness Codes\Drive Cycle (H4DO) - Procedure Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Drive Cycle>PROCEDURE > DRIVE CYCLE I.
Is DTC P2404 displayed on the display?
Yes
Replace the leak check valve assembly. See: Powertrain Management\Emission Control Systems\Leak Detection Valve\Specifications\Leak Check Valve Assembly (H4DO) - Specification Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES)(H4DO)>Leak Check Valve Assembly.
No

Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.

Note:

In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause.

1.OUTLINE OF DIAGNOSIS

Note:

For the detection standard, refer to DTC P0455. See: P0455\Engine System (H4DO) - Diagnostic Procedure w/ DTC Ref. to ENGINE (DIAGNOSTICS)(H4DO)>Diagnostic Procedure with Diagnostic Trouble Code (DTC)>DTC P0455 EVAP SYSTEM (CPC) LEAK DETECTED (LARGE LEAK).

 

Wow @cardoc ! That is an impressive write up! Much appreciate! I should print this out and just happen to leave it sitting on the dash when I bring it back to the dealer!
Looks like I have a date with my multi-meter tonight!

 

I didn't write that up, Subaru did. I just copy/pasted it here.

 

Either way, I REALLY appreciate the effort!

I got word today that the part is covered under my warranty so I will be off to the dealer next week for a repair!

 

Any idea what the dealer would charge for this repair? (I know the repair was covered by your warranty). I have the exact same condition (including rain / wet weather). 2016 Outback 103K
Thanks.

 

2020 Subaru Outback with only 7000 miles and I started getting the same code (P2404). I took it to the dealer and they were claiming it was the gas cap but after checking the gas cap religiously at each fill up and still getting the code I took it back convinced it was something else. After diagnosis they are now going to replace the leak detection pump which they say is bad. Hopefully this fixes it but I haven't got the car back so only time will tell. I'll post back the result.

 

Here's the update: They said it was the leak detection pump. But surprisingly, it is not a separate part but included as part of the canister. Anyway, they replaced the canister and, other than taking nearly 2 weeks to diagnose and fix the issue, the problem appears to be fixed, so far anyway, and I'm somewhat happy. I cringe to think what this would cost if it wasn't under warranty. If I get the code again I'll update again. If not, then no need to update as the situation would be stable.