Safety Warning

DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.

2022 Subaru Outback BaseFACTORY OEM SERVICE PROCEDURE

DTC U1604: Lost Communication With Dc/Dc Converter Module 1: Notes

1. -

DTC detecting condition:

Detected when two consecutive driving cycles with fault occur.

Wiring diagram:

ENGINE TYPE FB
Auto Start Stop Ref. to WIRING SYSTEM>AUTO START STOP>WIRING DIAGRAM>ENGINE TYPE FB .

Courtesy of SUBARU OF AMERICA, INC.

ENGINE TYPE FA
Auto Start Stop Ref. to WIRING SYSTEM>BACK-UP LIGHT SYSTEM>WIRING DIAGRAM>ENGINE TYPE FA .

Courtesy of SUBARU OF AMERICA, INC.

CAUTION:

Use the check board when measuring the ECM terminal voltage and resistance.
After servicing or replacing faulty parts, perform the last step of the basic diagnostic procedure. Ref. to AUTO START STOP (DIAGNOSTICS)>BASIC DIAGNOSTIC PROCEDURE .

CHECK DTC .
Read the DTC of [Engine] using the Subaru Select Monitor. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) . Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>DIAGNOSTIC TROUBLE CODE (DTC) .

Is DTC U0077 displayed?

Yes: Perform the diagnosis according to the DTC. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)>DTC U0077: LIN COMMUNICATION BUS "ECM/PCM" OFF . Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)>DTC U0077: LIN COMMUNICATION BUS "ECM/PCM" OFF .

No: Go to 2 .
CHECK HARNESS BETWEEN ECM AND DC/DC CONVERTER 1 .
1. Turn the ignition switch to OFF.
2. Disconnect the ECM connector.
3. Disconnect the DC/DC converter 1 connector.
4. Measure the resistance between ECM connector and DC/DC converter 1 connector.
  Connector and terminal
  
  ENGINE TYPE FB
  
  (B62) No. 17 - (B11) No. 9:
  
  ENGINE TYPE FA
  
  (B61) No. 56 - (B11) No. 9:
Is the resistance less than 1 Ω?

Yes: Go to 3 .

No: Repair the open circuit in harness between ECM connector and DC/DC converter 1 connector.
CHECK INPUT VOLTAGE OF DC/DC CONVERTER 1 .
1. Turn the ignition switch to ON. (With the connector disconnected)
2. Measure the voltage between DC/DC converter 1 connector and chassis ground.
  Connector and terminal
  
  (B11) No. 12 (+) - Chassis ground (-):
Is the voltage 12 V or more?

Yes: Go to 4 .

No: Repair the open harness or broken fuse of power supply circuit.
CHECK HARNESS BETWEEN DC/DC CONVERTER 1 AND CHASSIS GROUND .
1. Turn the ignition switch to OFF.
2. Measure the resistance between DC/DC converter 1 connector and chassis ground.
  Connector and terminal
  
  (B12) No. 4 - Chassis ground:
Is the resistance less than 5 Ω?

Yes: Go to 5 .

No: Repair the open circuit of harness between DC/DC converter 1 connector and chassis ground.
CHECK REPEATABILITY .
1. Using the Subaru Select Monitor, perform the clear memory of [Auto Start Stop]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
2. Execute the inspection mode. Ref. to AUTO START STOP (DIAGNOSTICS)>INSPECTION MODE .
Is DTC U1604 displayed?

Yes: Check for the poor contact in DC/DC converter 1 connector and ECM connector. Replace the DC/DC converter 1 if no fault is found. Ref. to STARTING/CHARGING SYSTEMS (H4DO)>AUTO START STOP DC/DC CONVERTER . Ref. to STARTING/CHARGING SYSTEMS (H4DOTC)>AUTO START STOP DC/DC CONVERTER .

No: 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.

Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.
• Brakes feel soft, pull hard to one side, or make grinding noises.
• The engine overheats, stalls repeatedly, or misfires under load.
• You are missing required tools, torque specs, or safe lifting equipment.
• You are not confident in the next step or safety outcome.

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