DTC U0028 Heater Control Panel Communication Error: Wiring Diagram

Auto A/C

Air Conditioning System. See appropriate SYSTEM WIRING DIAGRAM article.

Fig 1: Auto A/C - Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

Manual A/C

Air Conditioning System. See appropriate SYSTEM WIRING DIAGRAM article.

Fig 2: Manual A/C - Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

DIAGNOSTIC PROCEDURE

Step	Check	Yes	No
1 CHECK CONNECTOR. Check the power supply circuit connectors for poor contact.	Is there poor contact?	Repair the connector.	Go to step 2 .
2 CHECK FUSE. Turn the ignition switch to OFF. Remove the fuse No. 4 and No. 31 from the main fuse box or fuse & relay box. Check the condition of fuse.	Is the fuse blown out?	Replace the fuse.	Go to step 3 .
3 CHECK A/C CONTROL PANEL POWER CIRCUIT. Remove the A/C control panel. Disconnect the A/C control panel connector. Measure the voltage between A/C control panel connector terminal and chassis ground after turning the ignition switch to ON. Connector & terminal Auto A/C (i88) No. 5 (+) - Chassis ground (-): (i88) No. 6 (+) - Chassis ground (-): Manual A/C (i88) No. 1 (+) - Chassis ground (-): (i88) No. 7 (+) - Chassis ground (-):	Is the voltage 10 V or more?	Go to step 4 .	Check for open or short circuit in the harness between A/C control panel and fuse.
4 CHECK A/C CONTROL PANEL GROUND CIRCUIT. Measure the resistance of harness between A/C control panel and chassis ground after turning the ignition switch to OFF. Connector & terminal Auto A/C (i88) No. 12 - Chassis ground: Manual A/C (i88) No. 11 - Chassis ground:	Is the resistance less than 10 Ω?	Go to step 5 .	Repair the harness for ground line.
5 CHECK COMMUNICATION CIRCUIT HARNESS. Disconnect the connector from A/C control panel and A/C CM. Using a tester, check for continuity between the harness terminals. Connector & terminal Auto A/C (i80) No. 18 - (i88) No. 10: (i80) No. 7 - (i88) No. 4: Manual A/C (i80) No. 18 - (i88) No. 6: (i80) No. 7 - (i88) No. 12:	Is there continuity?	Go to step 6 .	Repair or replace the open circuit of harness.
6 CHECK HARNESS. Using a tester, check continuity between terminals. Connector & terminal (i80) No. 18 - No. 7: (i80) No. 18 - Chassis ground: (i80) No. 7 - Chassis ground:	Is there continuity?	Repair or replace the short circuit of the harness.	Replace the A/C control panel. Refer to REMOVAL .