The Xi'an Zhipangcun Air Conditioning Repair Department encountered a tricky issue with a Panasonic old-style fixed-frequency air conditioner. The unit wouldn't turn on, neither with the power on nor the emergency switch. The air conditioner was powered, the relay voltage was normal, the 7805 5V voltage was fine, and the tubes were okay, but it simply wouldn't start. Later, I heard from a colleague that the air conditioner wasn't properly repaired because the customer refused to let the mainboard be taken away for inspection and repair. That was the end of it.