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AOTA72LALT / AOTA90LALT – Intermittent 2E Fault
1. Typical Symptom
An AOTA72LALT or AOTA90LALT may intermittently stop and display a 2E fault. The fault may not be present when the technician attends site.
Typical reported behaviour includes:
- The unit operates normally for a period before faulting again.
- Resetting or cycling the power allows the system to operate again.
- The fault may be difficult to reproduce during a service visit.
- Voltage and system operation may appear normal when measurements are taken.
- Wiring looms and connections may show no obvious signs of being loose or damaged.
2. Field History
This troubleshooting guidance is based on field experience involving units with recurring 2E faults where significant component replacement had already taken place.
| Item | Example Field History |
|---|---|
| Fault | Intermittent 2E |
| Initial condition | Fault present from initial installation / commissioning |
| Inverter PCB | Replaced multiple times |
| Control PCB | Replaced |
| Contactor | Replaced |
| Electrical looms | Checked for loose connections with no obvious issue found |
| After reset | System resumes normal operation for a period |
3. Check the Incoming Three-Phase Supply
The AOTA72LALT and AOTA90LALT are three-phase systems. An intermittent incoming power supply issue should therefore be considered where the fault cannot be reproduced during normal testing.
Areas to investigate include:
- Momentary loss of one phase.
- Voltage sag under load.
- Voltage spikes or other supply disturbances.
- Phase voltage imbalance.
- Site load or phase loading conditions changing during the day.
- Loose or high-resistance connections upstream of the outdoor unit.
- Supply isolator, circuit breaker, terminals or other upstream electrical connections.
- Contactor operation and contact condition.
Static voltage readings
Measure and record the incoming supply across all phases and confirm that the readings are balanced and within the allowable supply range for the equipment.
However, a balanced voltage reading while the unit is operating normally only confirms the condition of the supply at the time the measurement was taken.
If all phases appear even during a service visit but the equipment only faults intermittently, the voltage reading does not eliminate the possibility of a supply issue occurring at another time.
DC circuit measurements
Where DC bus or capacitor voltages are being checked, measurements should be compared with the applicable service information and taken using appropriate electrical safety procedures.
A DC measurement taken while the system is operating correctly does not necessarily capture an intermittent disturbance occurring on the incoming AC supply.
4. Power Logging is Recommended
For a recurring intermittent 2E fault where the inverter PCB, control PCB and contactor have already been replaced, power logging should be completed before authorising further major component replacement.
The logger should be capable of identifying, where possible:
- Voltage on each phase.
- Short-duration voltage sags.
- Loss of phase.
- Phase imbalance.
- Overvoltage or undervoltage events.
- Supply interruptions.
- Timing of electrical events so they can be compared with the time the air conditioner faults.
Measurements taken several minutes or hours after the event may show a completely normal supply.
5. Recommended Diagnostic Procedure
| Step | Action |
|---|---|
| 1 | Confirm the recorded fault history and establish whether the same 2E fault has occurred repeatedly. |
| 2 | Review all components previously replaced. Avoid replacing the same components again without new diagnostic evidence. |
| 3 | Inspect power and control wiring, plugs, looms and terminals for loose, damaged, overheated or high-resistance connections. |
| 4 | Inspect the contactor and associated connections. Confirm correct operation. |
| 5 | Measure the incoming three-phase supply and record all readings. |
| 6 | Confirm phase balance and check the electrical supply under different operating loads. |
| 7 | Install a three-phase power quality logger. |
| 8 | Allow the logger to remain connected until the intermittent fault occurs again. |
| 9 | Compare the timestamp of the 2E fault with the logged electrical data. |
| 10 | Only proceed with additional component replacement where testing provides evidence supporting a component fault. |
6. Diagnostic Decision
| Finding | Recommended Direction |
|---|---|
| Voltage disturbance recorded at the same time as the fault | Investigate the incoming supply, switchboard, upstream connections and site electrical system. |
| Phase loss or significant imbalance recorded | Electrical supply issue requires investigation and rectification. |
| Supply remains stable when fault occurs | Continue equipment diagnostics using the applicable service information and recorded operating data. |
| No fault occurs during logging period | Continue monitoring where practical. Do not replace components solely because an intermittent fault has previously occurred. |
7. Information to Record
For intermittent cases, record as much information as possible before escalation:
- Outdoor unit model and serial number.
- Date and approximate time of each 2E occurrence.
- Whether power cycling restores operation.
- Components previously replaced.
- Incoming voltage measurements for all phases.
- Whether readings were taken while the system was under load.
- Any recorded phase imbalance.
- Power logger results covering the time of the fault.
- Condition of contactors, terminals, isolators and electrical connections.
- Any other equipment on site affected at approximately the same time.
A recurring 2E fault that survives multiple PCB and contactor replacements should not automatically result in further board replacement. Where the fault clears following a power reset and cannot be reproduced during attendance, an intermittent incoming electrical supply issue becomes an important part of the diagnosis. Power logging provides the evidence required to confirm or eliminate this possibility.
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