thermostat says it's calling for cool but my ac outdoor unit never turns on, just the inside fan runs.

Diagnosis

The Honeywell Home RTH9585WF is successfully triggering the "Cool On" state, which activates the G-wire to run the indoor blower fan, but the Y-wire signal—which tells the outdoor condenser to start—is not successfully completing the electrical circuit. Because the indoor fan is running, you know the thermostat has power (C-wire) and the internal relays are functioning. The failure is occurring in the 24VAC signal path between the thermostat's Y terminal and the contactor in the outdoor unit. This is typically caused by a "break" in the communication line. This could be a physical disconnection at the terminal block, a tripped 24V safety switch (such as a float switch in the condensate pan), or a blown fuse on the outdoor condenser's control board. If the thermostat display shows the cooling snowflake icon and the fan is blowing, but the outdoor compressor remains silent, the issue is almost certainly a loss of 24V power to the outdoor contactor coil.

How to Fix It

Follow these steps in order to isolate and repair the signal break:

1. Verify Thermostat Wiring: Pull the RTH9585WF off its wall plate. Inspect the yellow wire connected to the Y terminal. Ensure the wire is stripped back approximately 3/8 inch and is clamped tightly under the screw. If the wire is loose, tug it gently to ensure it is seated in the terminal and not just resting against the screw.
2. Check the Condensate Overflow Switch: Locate your indoor air handler. Look for a small plastic switch (float switch) installed in the PVC drain line or the emergency drain pan. If the pan is full of water, the switch will break the Y-wire circuit to prevent flooding. Clear the clog using a wet/dry vacuum on the exterior drain line to drop the float and restore the signal.
3. Inspect the Outdoor Disconnect: Go to the exterior disconnect box (the small metal box mounted to the wall near the AC unit). Open the cover and ensure the T-handle pullout is fully inserted or the circuit breaker is flipped to "ON." A partially seated pullout can cause intermittent power loss.
4. Test the Contactor: With the thermostat calling for cool, carefully remove the outdoor unit's service panel. Look at the contactor (the relay with two large wires and two small wires). If you hear a humming sound but the unit isn't starting, the contactor may be pitted. If there is no sound, use a multimeter to check for 24VAC across the two small side terminals of the contactor. If you have 24V but the unit isn't running, the contactor has failed and needs replacement.

If That Didn't Work

• Blown Control Board Fuse: Many outdoor units have a 3-amp or 5-amp automotive-style fuse on the control board. If this fuse blows due to a power surge, the Y-signal will reach the unit, but the board won't trigger the compressor. Replace the fuse with an identical amperage rating.
• Capacitor Failure: If the outdoor unit makes a loud "buzzing" or "clicking" sound every few minutes but the fan/compressor never starts, the dual-run capacitor (the silver cylinder) has likely failed. This prevents the motor from kicking over despite receiving the signal from the RTH9585WF.
• C-Wire Voltage Drop: Use a voltmeter to check the voltage between the R and C terminals at the thermostat. If it is below 24VAC, the thermostat may have enough power to run the screen and the G-fan relay, but not enough to push the Y-signal through the long wire run to the outdoor unit.
• Faulty Y-Relay: Although rare, the internal relay for the Y-terminal on the RTH9585WF can fail. To test this, turn off the power, remove the thermostat, and jump the R wire directly to the Y wire using a small piece of insulated copper. If the outdoor unit starts, the thermostat hardware is defective and requires replacement.

Heads Up

• Warning: High voltage (240V) is present in the outdoor disconnect and condenser. Always turn off the breaker before touching the contactor or capacitor.
• Capacitor Danger: Capacitors store electrical energy even when the power is off. Discharge them using a resistor or an insulated screwdriver before touching the terminals.
• Wire Color Standards: While yellow is the standard for Y (cooling), always verify the labels at the air handler board rather than relying solely on wire colors.
• Drain Maintenance: To prevent future outages, pour a cup of distilled white vinegar down your condensate drain line every 6 months to prevent algae clogs from triggering the float switch.