Summary

Use this checklist to isolate and resolve a CO communication fault on a wired wall controller. The sequence below verifies power, network integrity, addressing, and component compatibility before replacement is considered.

Applies to

  • Wired wall controllers on residential/VRF indoor units

  • Service technicians diagnosing communication (“CO”) alarms

Safety

  • Isolate mains supply before opening indoor unit panels or handling wiring.

  • Use insulated probes and measure DC voltages on the correct meter range.

  • Restore power only when directed for live tests.

Tools

  • Digital multimeter (V DC, V AC)

  • Small flathead screwdriver for DIP switches

  • Controller/indoor PCB part numbers and model labels for cross-checks

Procedure

  1. Confirm controller power

    • Measure 12 V DC at the wall controller terminals.

    • If 12 V DC is missing, go to Step 5.

  2. Verify cabling

    • Confirm screened data cable ~0.33 mm² conductors (commonly called “data cable”).

    • Check for damage, poor terminations, reversed polarity, or shorts to shield/earth.

  3. Standalone vs networked

    • Determine if the controller is standalone. If networked, temporarily isolate to standalone for testing, or ensure addressing is correct before proceeding.

  4. DIP switch and address settings

    • Confirm DIP switch positions on both the wall controller and the indoor control PCB per site design (address/group/mode).

    • Correct any mismatch.

  5. Upstream power check (if no 12 V DC at controller)

    • Verify indoor unit mains 240 V AC is present.

    • Confirm indoor PCB DC supply ~18 V DC (where applicable) feeding the controller regulator.

    • If upstream power is correct but the controller still lacks 12 V DC, replace the indoor control PCB (or the controller power module as applicable).

  6. Signal integrity (live test)

    • With power on, measure communication line DC between the wall controller and indoor PCB.

    • Expect ~7–11 V DC on both sides of the link.

    • Significant deviation on either side indicates a fault toward that side (controller, cable segment, or indoor PCB interface).

  7. Isolate the weak side and replace as needed

    • Based on Step 6 readings, replace the component associated with the side showing poor voltage or unstable signal after confirming wiring and terminations.

  8. Part/model compatibility

    • After any replacement, confirm part numbers and indoor model numbers are a compatible pairing.

    • Do not rely solely on the error code—verify against parts documentation.

  9. Validate the test method

    • If all measurements and settings appear correct yet the CO fault persists, re-check probe points, polarity, and meter range. Repeat Steps 1–7 to confirm results.

  10. Reset to clear latched errors

  • After correcting any issue, power the system OFF for at least 10 minutes to allow full discharge and fault reset before retesting.

Quick decision guide

  • No 12 V DC at controller → Check 240 V AC & ~18 V DC on indoor PCB → If present, replace indoor PCB power to controller.

  • 7–11 V DC present on both sides but CO remains → Re-check addressing/DIP and cabling; verify compatibility; reset power for 10 minutes.

  • Asymmetric or out-of-range comm voltage → Fault lies toward the side with poor reading → Inspect/replace that component or cable.

  • Post-replacement CO → Confirm correct part number and indoor model pairing; re-address and retest.

Technician notes

  • Keep cable screens continuous and terminated per site standard; avoid sharing with high-noise conductors.

  • Label addresses at the controller and indoor PCB after final setup to simplify future service.

  • Document final measurements (12 V DC feed and comm line V DC) on the job card for traceability.