General Assist Technical Reference
PFC PCB Testing Procedure
Power Factor Correction PCB diode-test procedure
Applicable models: AOTD**LATT, AOTD**LBTT and AOTG**LATT
The Power Factor Correction (PFC) PCB regulates current flow within the inverter power circuit and improves electrical efficiency. This procedure provides a structured method for checking the board for an internal short circuit using a multimeter in diode-test mode.
Danger — high voltage: This test must only be completed by a qualified service technician. Isolate and lock out the electrical supply, verify that power is off and allow the DC capacitors to discharge before touching or disconnecting the PCB. Confirm zero voltage with a suitable meter before testing.
1. Symptoms that may indicate PFC PCB failure
- The circuit breaker trips immediately when the system is energised.
- There is no power supply to the control PCB.
- The PFC PCB has visible damage, discolouration or burnt components.
Important: These symptoms do not confirm a failed PFC PCB by themselves. Inspect the complete power circuit and wiring before condemning the board.
2. Test equipment and preparation
| Requirement | Action |
|---|---|
| Multimeter | Use a reliable digital multimeter with a diode-check function. |
| Power isolation | Isolate and lock out the outdoor unit supply. Verify that no voltage is present. |
| Capacitor discharge | Allow the inverter capacitors to discharge and confirm the DC bus is at a safe level before proceeding. |
| PCB isolation | Disconnect the PFC PCB from the circuit so parallel components do not influence the readings. |
| Visual inspection | Check for burnt components, cracked solder joints, damaged terminals and contamination. |
PFC testing is similar to IPM diode testing; however, the choke coils in the PFC circuit can make readings more sensitive. Use firm probe contact and allow the meter reading to stabilise.
3. Test points
| Terminal | Identification |
|---|---|
| TM504 | Positive test terminal — orange terminal in the source diagram. |
| TM505 | Negative test terminal — purple terminal in the source diagram. |
| TM500–TM503 | PFC switching and choke-coil circuit test points. |
Use the PCB terminal markings rather than relying only on wire or terminal colour. Board layout and component location can vary between model revisions.
4. Diode-test procedure
- Confirm the power supply is isolated and the capacitors are discharged.
- Disconnect the PFC PCB from the circuit.
- Set the multimeter to diode-test mode.
- Apply the meter leads exactly as shown in the table below.
- Allow each reading to stabilise, then record the result.
- Compare all results with the expected values before determining board condition.
| Red lead | Black lead | Expected reading |
|---|---|---|
| TM504 | TM500 | OL |
| TM504 | TM501 | OL |
| TM504 | TM502 | OL |
| TM505 | TM500 | Approximately 0.4 V |
| TM505 | TM501 | Approximately 0.4 V |
| TM505 | TM502 | Approximately 0.4 V |
| TM501 | TM504 | Approximately 0.4 V |
| TM502 | TM504 | Approximately 0.4 V |
| TM503 | TM504 | Approximately 0.4 V |
| TM502 | TM505 | OL |
| TM503 | TM505 | OL |
OL means the meter indicates an open circuit or no forward conduction in that test direction. The exact display may appear as OL, O/L, 1 or an over-range symbol depending on the meter.
5. Results and interpretation
| Result | Interpretation | Action |
|---|---|---|
| Readings match the table | The diode paths are behaving as expected. | Continue diagnosis of the power circuit if the original fault remains. |
| 0.0 V or near-zero reading | Indicates a likely short circuit in the tested path. | Do not re-energise. Replace the PFC PCB after confirming the board is isolated from the surrounding circuit. |
| Unexpected OL or abnormal voltage | May indicate an open semiconductor path, poor probe contact or influence from connected components. | Repeat the test, confirm lead polarity and PCB isolation, then compare with the model-specific service information. |
Do not re-energise the system with a confirmed shorted PFC PCB. Before fitting a replacement board, check the inverter module, diode bridge, compressor circuit, fan motors, reactors/choke coils and wiring for additional faults that may damage the new PCB.
6. Final checks before re-energising
- Confirm all plugs, power terminals and earth connections are correctly refitted.
- Confirm no tools, loose hardware or test leads remain inside the outdoor unit.
- Check that replacement PCB settings, connectors and harness routing match the original board.
- Refit all electrical covers and guards before restoring power.
- Monitor start-up current and operation after energising the unit.
Summary: Correct PFC PCB testing requires complete electrical isolation, capacitor discharge, accurate lead placement and comparison of every diode reading. A confirmed 0.0 V short-circuit result indicates that the PFC PCB must be replaced before the system is re-energised.
This information is intended for qualified refrigeration and air-conditioning service personnel. Always follow the model-specific service manual, electrical safety procedures and applicable workplace requirements.