My AC has been working fine. But today when I tried to turn on my AC after I had come back from church, I found my ground floor ac was not cooling but with blowing house temperature wind only. The ac upstairs works just fine. When I checked both ac control panels (one in the ground floor, one on the 2nd floor), both indicated the ACs are running with fans automatically on. After about 3-4 hours, the temperature on the 2nd flood dropped significantly to also set temperature, but the temperature on the ground floor barely change (dropped just 1-2 degrees which are mostly likely due to the cooling air flowing from the 2nd floor to the ground as the are openly connected in the same two-story house. So, I decided to check the 2 AC units outside of the house. There I noticed the unit for the upstairs were working hard blowing warm airs, but the unit for the ground floor was not running at all. A very careful check and listening, I found the AC for the ground floor seems to try to run [with some clicks regularly heard] but did not start. Can any one advise what could be the cause? Many thanks!

Most likely need a new start capacitor.

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Thank you very much for pin-point the problem. Do I need to buy a capacitor specific for my ac unit (model)? In fact, I even don't know where the capacitor locates? Appreciate your further instructions.

If it is the capacitor, most likely the top where the wires are will be swolen or bubbled out. It is supposed to be flat like the pic. The capacitor is located in the pannel where the electric goes in the outdoor unit. Make sure you turn the breaker off. There will be 4 wires 2 on c 1 on fan and 1 on hurm. Write down what wires went where and take the cap to any hvac store they will test it and get you a new one.

Thank you for the instructions. I will try to see if I can do it myself.