A reading of 0 volts between Y and the coil input on line #11 under thermostat closed and fan auto indicates which condition?

• A. The coil is energized
• B. The circuit is shorted
• C. The coil is de-energized
• D. The thermostat is defective

Answer: (D)

When the cooling call is active, the thermostat must close the Y circuit and deliver 24 VAC from the R supply to the outdoor coil input. A reading of zero volts between Y and the coil input means no voltage is reaching the coil, so the coil won’t energize. That points to the thermostat not making the Y connection when it should, i.e., the thermostat is defective. If the thermostat were functioning, closing the cooling circuit would produce a voltage across Y and the coil input, energizing the outdoor coil. Other faults (like a short or a broken wire) would typically show different symptoms or readings, so the lack of voltage here most strongly indicates a defective thermostat.