DTC U0028: Heater Control Panel Communication Error

DTC detecting condition: 

UART communication error.

Trouble symptom: 

Unable to operate A/C.

Wiring diagram: 

Air conditioning system Ref. to WIRING SYSTEM (CROSSTREK MODEL (HEV))>AIR CONDITIONING SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK CONNECTOR  .

   Check poor contact of the power supply circuit connectors, A/C control panel connector and A/C control module connector.

   Is the check result OK?

   Yes:  Go to  2.

   No:  Repair the connector.

2. CHECK FUSE  .
   1. Turn the ignition switch to OFF.
   2. Remove the fuses No. 8 and 14 from fuse and relay box.
   3. Check the condition of fuse.

   Is the check result OK?

   Yes:  Go to  3.

   No:  Replace the fuse. If the replaced fuse blows out easily, repair the ground short circuit in harness between the fuse - A/C control panel.

3. CHECK A/C CONTROL PANEL POWER SUPPLY CIRCUIT (OPEN OR SHORT)  .
   1. Remove the A/C control panel. Ref. to AIR CONDITIONER (HEV)>CONTROL PANEL .
   2. Disconnect the A/C control panel connector.
   3. Turn the ignition switch to ON.
   4. Measure the voltage between A/C control panel connector terminal and chassis ground.

      Connector and terminal 

      (i80) No. 2 (+) - Chassis ground (-):

      (i329) No. 4 (+) - Chassis ground (-):

   Is the voltage 10 V or more?

   Yes:  Go to  4.

   No:  Check for open or short circuit in the harness between A/C control panel and fuse.

4. CHECK A/C CONTROL PANEL GROUND CIRCUIT (OPEN)  .
   1. Turn the ignition switch to OFF.
   2. Measure the resistance of harness between A/C control panel and chassis ground.

      Connector and terminal 

      (i329) No. 5 - Chassis ground:

   Is the resistance less than 10 Ω?

   Yes:  Go to  5.

   No:  Repair the harness for ground line.

5. CHECK COMMUNICATION CIRCUIT HARNESS (OPEN CIRCUIT)  .
   1. Disconnect the connector from A/C control panel and A/C control module.
   2. Using a tester, check for continuity between the harness terminals.

      Connector and terminal 

      (i80) No. 19 - (i329) No. 6:

      (i80) No. 20 - (i329) No. 7:

   Is there continuity?

   Yes:  Go to  6.

   No:  Repair or replace the open circuit of harness.

6. CHECK COMMUNICATION CIRCUIT HARNESS (SHORTED BETWEEN LINES AND SHORTED TO GROUND)  .

   Using a tester, check continuity between terminals.

   Connector and terminal 

   (i80) No. 19 - (i80) No. 20:

   (i80) No. 19 - Chassis ground:

   (i80) No. 20 - Chassis ground:

   Is there continuity?

   Yes:  Repair or replace the short circuit of the harness.

   No:  Replace the A/C control panel. Ref. to AIR CONDITIONER (HEV)>CONTROL PANEL .