To help air-conditioning contractors diagnose problems with inverter-controlled air conditioners, EDC International offers an instrument that can identify whether the compressor or inverter is faulty. To use the Inverter Phase Check, the supply is disconnected from the compressor and connected to the module. When the inverter is switched on, the six lights should light up — red, yellow and blue. If the inverter is faulty, at least one of these lights will not illuminate, indicating the position of the fault.