To prepare you for this month’s troubleshooting situation … you’re not the first technician to be called, nor are you the second technician. You’re the third one that’s been called in to solve this customer’s problem, which involves a heat pump that serves a residential building in a mild Southwest desert climate.

The first technician on this job correctly diagnosed a failed printed circuit board, but the customer declined the repair. The customer then called your service company and requested replacement of the board, and also opted for a new programmable thermostat, and the second technician installed both components. The reason you’re following up on this repair is because the customer reported that while the unit is now operational, the indoor fan motor occasionally comes on and causes the temperature in the home to rise above the thermostat set-point. When the technician who replaced the parts returned to test the equipment operation, the customer couldn’t provide any information beyond what’s explained above. At that time, the test procedures were implemented to cycle the unit in both the heating and cooling modes, and it performed normally.

Due to the advanced age of this equipment, you do the necessary research before you proceed with the service call, and you find the wiring diagram (minus the supplemental heat strips this unit employs) shown in Figure 1. The legend for it is shown in Figure 2.