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.
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:
Auto Start Stop Ref. to WIRING SYSTEM (CROSSTREK MODEL (EXCEPT FOR HEV))>AUTO START STOP .
- 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 2.5)>DATA MONITOR . Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.0 (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC U0077 displayed?
Yes: Perform the diagnosis according to the DTC. Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.5)>DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)>DTC U0077: LIN COMMUNICATION BUS "ECM/PCM" OFF . Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.0 (EXCEPT FOR HEV))>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
.
- Turn the ignition switch to OFF.
- Disconnect the ECM connector.
- Disconnect the DC/DC converter 1 connector.
- Measure the resistance between ECM connector and DC/DC converter 1 connector.
Connector and terminal
(B134) No. 17 - (B562) 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
.
- Turn the ignition switch to ON. (With the connector disconnected)
- Measure the voltage between DC/DC converter 1 connector and chassis ground.
Connector and terminal
(B562) 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
.
- Turn the ignition switch to OFF.
- Measure the resistance between DC/DC converter 1 connector and chassis ground.
Connector and terminal
(B563) 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
.
- Using the Subaru Select Monitor, perform the clear memory of [Auto Start Stop]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
- 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 (EXCEPT FOR HEV))>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.
NO RELATED
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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.