1. Symptom / complaint

• Unit presents Error 13 intermittently with no consistent time pattern. The fault may occur after hours, days, or within minutes.
• Error may persist despite replacement of the indoor PCB(s) and outdoor control PCB.
• Basic cable checks may appear acceptable, including continuity and insulation-resistance tests, while the fault continues.

2. Most common root causes

2.1 Interconnect cable or joint issues

• Water in a junction box, trunking join, or corrugated conduit can create intermittent leakage or voltage instability.
• Field examples include cuts or nicks across neutral and signal conductors, conduit containing water, and junctions concealed in walls or roof spaces.

2.2 Loose or fatigued indoor harness connections

• PCB-to-PCB harnesses can become marginal due to loose pins, partial engagement, pin back-out, poor crimping, or conductor breakage near strain relief.

2.3 Conductor damage not detected by static testing

• Continuity and insulation-resistance tests can pass while the cable is stationary and de-energised. Under vibration or thermal movement, damaged conductors can open or short momentarily.

2.4 Associated indoor electrical-load instability

• Indoor fan motor(s) operating outside specification may introduce abnormal load conditions or interference.
• Capacitor or DC-bus stability can be checked as an advanced diagnostic only where the correct method, tooling, and competency are available.

3. Diagnostic workflow - recommended order

Step 1 - Visual and physical inspection

1. Inspect the indoor-outdoor interconnect at both terminal points for loose screws, stray strands, heat discolouration, corrosion, or green copper oxide.
2. Inspect every junction point between the indoor and outdoor units, including roof-space junction boxes, trunking joins, conduit transitions, and concealed wall cavities.
3. Ask the installer whether any concealed joints exist along the cable path.
4. With power isolated, gently tug each conductor at plug and terminal entries to identify an internal break at the clamp or strain-relief point.

Step 2 - Reseat and secure indoor harness connections

1. Locate all indoor PCB-to-PCB harnesses.
2. Unplug and reconnect each connector fully. Confirm positive engagement and locking-tab operation.
3. Inspect pins for spread terminals, pin push-back, oxidation, poor crimps, and conductor damage.
4. If available, substitute a known-good harness to validate the connection.

Step 3 - Check communication voltages

Measure and record DC communication voltage at both ends of the interconnect and at the cable/termination points.

Step 4 - Prove or disprove the interconnect cable

If the interconnect is concealed inside a wall or cannot be easily replaced, run a temporary surface interconnect between the indoor and outdoor units where safe and approved.

• If Error 13 stops, treat the interconnect cable path, junctions, moisture ingress, or conductor damage as the likely root cause even if previous continuity and insulation-resistance tests passed.
• Return the issue to the installer to locate and rectify concealed junctions or replace the cable correctly.

Step 5 - Secondary checks

• Indoor fan motor(s): Confirm correct operation and check for abnormal current draw or intermittent stalling.
• Capacitor / DC bus stability: Perform only when the cable and harnesses have been proven sound and the technician has the correct procedure, tooling, and competency.

4. Corrective actions based on findings

5. Technician case documentation

Record the following before closing or escalating the case:

• Frequency and pattern of Error 13 occurrences.
• Whether indoor or outdoor PCB(s) were replaced, including part numbers if available.
• Communication voltages measured at outdoor terminal 2-3, outdoor terminal 2 and cable, indoor terminal 2-3, and indoor terminal 2 and cable.
• Confirmation of any junction boxes or hidden joins along the interconnect path.
• Result of the temporary interconnect test - Pass or Fail.