If the IFR operates and the fan starts when the thermostat fan switch is set to On, and the relay operates when the switch is set to Auto, the most likely cause is a defective ___________.

• A. thermostat
• B. blower motor
• C. capacitor
• D. relay

Answer: (A)

In this scenario, you’re testing how the fan is commanded in two different thermostat settings. When the thermostat fan switch is set to On, the blower starts, which shows the blower motor, wiring, and the relay circuit for the blower are functioning. When the switch is set to Auto, the thermostat must energize the blower relay only when there’s a heating or cooling call. If the relay does engage in Auto but the fan still doesn’t run, the problem is in the thermostat’s Auto fan control or its wiring, not in the blower circuit itself. Other components—like a bad blower motor or a failing capacitor—would typically cause the fan to fail in both On and Auto modes, and a faulty relay would affect both modes or the control path to the relay. Since On works but Auto doesn’t point to a defective thermostat.