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.

CAUTION: Before performing diagnosis, refer to "CAUTION" in "General Description". Ref. to AIR CONDITIONER (DIAGNOSTICS) (HEV)>GENERAL DESCRIPTION>CAUTION .

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.
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.
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.
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.
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.
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 .