Applies to

  • AOTA72LALT

  • AOTA90LALT

Symptom / Complaint

  • Unit presents Error 13 intermittently with no consistent time pattern (may occur after hours, days, or within minutes).

  • Error may persist despite replacement of:

    • Indoor PCB(s)

    • Outdoor control PCB

  • Basic cable checks may appear “OK” (continuity and insulation resistance pass) yet the issue continues.

What Error 13 Typically Indicates

Error 13 is commonly associated with communication instability between indoor and outdoor units (and/or between indoor PCBs where applicable). When intermittent, the root cause is frequently an intermittent connection, moisture ingress, or cable integrity issue under load/movement rather than a permanently open circuit.

Safety & Access

  1. Isolate power before handling wiring/PCB connections.

  2. Confirm correct lock-out/tag-out procedure as per site requirements.

  3. Visually inspect all terminations for signs of heat, corrosion, moisture, or conductor damage.

Most Common Root Causes (Intermittent Cases)

1) Interconnect cable/joint issues (often moisture-related)

  • Water in a junction box, trunking join, or corrugated conduit can create intermittent leakage/voltage instability.

  • Field examples include:

    • Cuts/nicks across neutral and signal conductors inside conduit

    • Conduit found full of water

    • Junctions hidden in walls/roof spaces

2) Loose / fatigued plugs or indoor harness links between PCBs

  • Harnesses linking indoor boards can be marginal: loose pins, partial engagement, pin back-out, conductor break at strain relief.

3) Conductor damage not detected by static tests

  • Continuity and IR tests may pass when the cable is stationary and de-energised. Under vibration/thermal movement, damaged conductors can open/short momentarily.

4) Associated indoor electrical load instability (less common)

  • Reports suggest checking:

    • Indoor fan motor(s) out of specification (creating noise/interference or abnormal load conditions)

    • Capacitor/DC bus stability (advanced check; proceed only if you have the correct method, tooling, and competency)

Diagnostic Workflow (Recommended Order)

Step 1 – Visual & Physical Inspection (Highest Yield)

  1. Inspect indoor-outdoor interconnect at both terminals for:

    • Loose screws

    • Stray strands

    • Heat discolouration

    • Corrosion/green copper oxide

  2. Inspect any junction points between units:

    • Roof space junction boxes

    • Trunking joins

    • Conduit transitions

    • Wall cavities (ask installer if any concealed joints exist)

✅ Tug test: With power isolated, gently tug each conductor at plug/terminal entries to confirm no internal conductor break at the clamp/strain relief.

Photo reference (typical issue):

  • Damaged conductor / corroded joint can intermittently fail under vibration/load (example matches image provided: exposed conductor, corrosion, compromised insulation).

Step 2 – Reseat & Secure Harness Connections (Indoor)

  1. Locate all indoor PCB-to-PCB harnesses.

  2. Unplug and re-plug each connector fully. Confirm positive engagement/locking tabs.

  3. Inspect pins for:

    • Spread terminals

    • Pin push-back

    • Oxidation

    • Poor crimp

If available, substitute known-good harness(es) to validate.

Step 3 – Check Communication Voltages (Record Readings)

Measure DC comms voltage at both ends and at cable.

Example field readings (for reference only; record actual values found):

  • Outdoor terminal 2–3: ~20–101 VDC

  • Outdoor “2 & cable”: ~52–61 VDC

  • Indoor terminal 2–3: ~88–121 VDC

  • Indoor “2 & cable”: ~117–131 VDC

Interpretation (general guidance):

  • Wide fluctuation can be normal depending on system state, but instability that correlates with the fault is significant.

  • A difference between “terminal” and “terminal & cable” measurements may suggest a termination/contact issue or cable/joint anomaly.

Step 4 – Prove/Disprove the Interconnect Cable (Critical for Intermittent Faults)

If the interconnect runs inside a wall or cannot be easily replaced:

✅ Run a temporary surface interconnect (where safe/approved) between indoor and outdoor units.

  • If Error 13 stops: the root cause is interconnect cable path/junction/moisture/damage even if IR/continuity previously passed.

  • Hand off to installer to locate and remediate concealed junctions or replace cable properly.

Important: Static insulation resistance may not reveal a moisture issue that becomes active under operating voltage, load, or environmental conditions.

Step 5 – Secondary Checks (If Cable/Harness Proven OK)

  1. Indoor fan motor(s) inspection / specification check

    • Confirm correct operation and no abnormal current draw or intermittent stall.

  2. Capacitor / DC bus stability checks (advanced)

    • Only perform if you have the correct procedure and tooling; document results clearly.

Corrective Actions (Based on Findings)

  • Moisture/junction issue found:

    • Dry out, remake joints, re-terminate, replace corroded components, improve sealing/IP rating, reroute to avoid water traps.

  • Cable damage found (nicks/cuts/crushed/green copper oxide):

    • Replace interconnect run end-to-end where possible. Avoid hidden joints.

  • Loose harness/plug issues:

    • Replace harness, repair terminal/pin seating, ensure strain relief and secure routing.

  • Temporary cable proves fault is in building wiring:

    • Return to installer to rectify concealed junctions/trunking/conduit water ingress and replace cable.

Notes for Case Documentation (Technician to Record)

  • Frequency/pattern of Error 13 occurrences

  • Whether indoor/outdoor PCB(s) were replaced (and part numbers if available)

  • Comms voltages measured at:

    • Outdoor terminal 2–3

    • Outdoor terminal 2 & cable

    • Indoor terminal 2–3

    • Indoor terminal 2 & cable

  • Confirmation of any junction boxes / hidden joins

  • Result of temporary interconnect test (Pass/Fail)