Example embodiments of the present disclosure relate to a heat pump including a system or method for detecting a fault in the heat pump. Some embodiments include a method for detecting a switch over valve fault where the heat pump includes a refrigerant cycle, a compressor, a first heat exchanger, a second heat exchanger, and a switch over valve, and the method includes operating the HVAC system in one of a heating mode or a cooling mode, monitoring first, second, and third refrigerant parameters associated with the refrigerant circuit using first, second, and third refrigerant sensors, determining first and second refrigerant inputs based on the first, second, and third second refrigerant parameters, and determining a refrigerant circulation mode by comparing the first refrigerant input to the second refrigerant input to provide an indication of whether the refrigerant is circulating in the heating mode or the cooling mode.