GENERAL AIRSTAGE | Technical Support

INV Communication Error - Outdoor Unit

Applicable models: ARTC72LATU / ARTC90LATU AOTA72LALT AOTA90LALT  |   Reference: Service Instructions, Troubleshooting 17, p. 02-19

Fault description: Communication is interrupted between the outdoor Main PCB and Inverter PCB. The manual also identifies a detection condition where the voltage difference across the inverter PCB cement resistor reaches 18 V or above on two consecutive detections. Confirm the displayed fault against the applicable model error-code table.
Electrical safety: For qualified electrical/refrigeration service personnel only. High voltage and stored charge can remain within inverter equipment after isolation. Follow lockout/tagout procedures, confirm absence of voltage and observe the manufacturer's DC-bus discharge instructions before disconnecting connectors or measuring resistance. Energised measurements must only be undertaken by authorised persons using appropriate instruments and safe work methods. Never bypass a safety device or manually force a contactor closed.

Likely causes

  • Loose, disconnected, incorrect or open-circuit wiring between control components.
  • Abnormal cement resistor resistance or associated inverter circuitry.
  • Outdoor Main PCB or Inverter PCB failure.
  • Additional field checks: Incorrect or unstable incoming supply, phase loss, fuse/protection issue, and a magnetic relay/contactor with faulty contacts or welded opened/closed.
Diagnostic order: Incoming power → magnetic relay/contactor → PCB wiring/connectors → cement resistor → PCB diagnosis. The first two checks are supplementary field diagnostics, not steps listed in the referenced troubleshooting chart.

Step 1 - Verify incoming power supply

  1. Confirm that the outdoor-unit isolator, breakers and appropriate upstream protective devices are operational and that the supply matches the unit nameplate and wiring diagram.
  2. Where applicable, measure phase-to-phase voltages and check for missing phases, imbalance or abnormal voltage variation. Verify supply voltage during the operating/starting condition if safely achievable.
  3. Inspect supply terminals, cable connections, fuses, disconnects and control-circuit feeds for looseness, overheating, corrosion or loss of continuity.
  4. Where a contactor switches supply to the inverter/control circuit, establish that correct voltage is available at both the supply side and the load side when the contactor is legitimately commanded closed.

If abnormal: Correct the power-supply or protective-device fault before diagnosing or replacing electronic boards. Do not assume that the presence of an error code proves a failed PCB.

Step 2 - Inspect the magnetic relay / contactor

Confirm the actual contactor arrangement from the unit wiring diagram. A contactor can fail either welded closed or stuck open, and can also have high-resistance or intermittent contacts.

  • Welded closed: After safe isolation, verify the contacts are not mechanically fused closed. A welded contactor may leave a downstream circuit energised even when the coil is not commanded on.
  • Stuck open / failure to close: Confirm the coil receives its specified control voltage when commanded and that the contactor actuates correctly. Check for a failed coil, damaged mechanism or open contacts.
  • Burnt or pitted contacts: Inspect for heat damage, discolouration or abnormal contact resistance that could cause voltage drop under load.
  • Incorrect operation: Check that the contacts open and close in response to the correct control signal and that load-side voltage corresponds with the commanded state. Check any auxiliary contacts where provided.
Important: Verify isolation on both sides of the contactor before touching conductors. A welded contact does not provide electrical isolation. Replace an electrically or mechanically defective contactor with the correct approved component; do not file, bridge or bypass its contacts.

Step 3 - Check control-unit connections

Manufacturer Check Point 1. With power isolated and stored energy discharged:

  • Check for loose terminal connections.
  • Check for removed or partially seated connectors.
  • Check for incorrect connector positions or miswiring.
  • Inspect the harness between Main PCB and Inverter PCB for damaged insulation, broken conductors or open circuits.
  • Inspect pins and connectors for contamination, corrosion, heat damage and poor retention.

If corrected: Reassemble correctly, restore power using the approved procedure, reset the fault and confirm whether normal operation and communication return.

Step 4 - Test the cement resistor

Manufacturer Check Point 2. Disconnect the relevant connector as directed by the model service instructions, then measure each cement resistor with a suitable ohmmeter under fully de-energised conditions.

Manual's nominal resistance: 5.6 Ω. The scan appears to print a tolerance of ± 5 Ω, which is unusual for this component.  Confirm the permissible tolerance from an authoritative component marking, wiring diagram or current manufacturer documentation before judging a reading. 
  • Check for open circuit, short circuit or a resistance outside the confirmed permitted range.
  • If the resistor is defective, replace it with the correct specified component and inspect for underlying causes of the failure.
  • Do not measure resistance through an energised circuit or across connected parallel circuitry when the procedure requires disconnection.

Step 5 - Diagnose the Main PCB and Inverter PCB

Manufacturer Check Point 3. If control wiring and cement resistor checks do not restore operation, the manual directs replacement of the Main PCB and Inverter PCB. Before proceeding, also confirm that the supply and any magnetic relay/contactor faults identified in Steps 1-2 have been ruled out.

Follow the applicable service/parts documentation for replacement sequence, compatibility, settings and post-replacement tests. Avoid replacing boards solely because the fault recurs without documenting the supply, contactor and wiring test results.

Quick diagnostic interpretation

FindingRecommended action
Missing / unstable supply voltage or phaseResolve the supply, protection or connection issue first.
Contactor remains closed with coil de-energisedInvestigate welded contacts; safely isolate and replace if confirmed.
Contactor fails to close when correctly commandedCheck control voltage, coil, mechanism and contacts.
Open / misconnected communication harnessRepair or reseat; restore power and verify.
Cement resistor outside verified toleranceReplace with the specified component and investigate cause.
All preceding checks pass; fault persistsProceed with manufacturer-directed Main PCB / Inverter PCB diagnosis and replacement.

Before closing the service job

Record the fault indication, supply voltage/phase readings, contactor commanded and actual states, connector findings, measured resistor values, repairs undertaken and final operating test. Confirm that the original communication error does not return after a normal restart.

Technical source: ARTC72/90LATU Service Instructions, Troubleshooting 17, page 02-19. Power-supply and magnetic relay/contactor checks are additional diagnostic guidance and are not shown in that source flowchart. Use the latest model-specific wiring diagram and service information.