A controller of an HVAC system is communicatively coupled to a suction-side sensor and a shutoff switch. The controller stores measurements of the suction-side property over an initial period of time. The controller detects that the shutoff switch is tripped at a first time stamp corresponding to an end of the initial period of time. The controller accesses the measurements of the suction-side property. The controller determines, based on the measurements of the suction-side property, whether the suction-side property has an increasing or decreasing trend. In response to determining that the suction-side property has the increasing trend, the controller determines that a malfunction of a fan caused the shutoff switch to trip. In response to determining that the suction-side property has the decreasing trend, the controller determines that a blockage of the refrigerant conduit subsystem caused the shutoff switch to trip.