DIAGNOSIS The compressor is not receiving the signal to start. This is usually due to the humidistat setting, a failed start capacitor, or a locked compressor.
FIX Set the humidistat control to its lowest setting (like 35% RH) and the fan speed to High. This gives the clearest 'on' signal. Listen closely for a click and a hum about 3 minutes after startup. If you hear a click but no hum, the start capacitor on the compressor is likely dead.
IF THAT DIDN'T WORK With the unit unplugged, you can check the start capacitor. It's a black cylindrical component near the compressor. Look for bulging or leaking oil. Replacing it requires discharging it and matching the microfarad (µF) and voltage rating exactly.
HEADS UP Compressor capacitors store a dangerous electrical charge even when unplugged. Do not touch the terminals unless you are trained to safely discharge them. Consider a professional repair.