honeywell t9 says it's in cool mode but the ac isn't kicking on

Diagnosis

The Honeywell Home T9 (RCHT9610WFSW2003) indicating "Cool" mode without the AC engaging typically points to a break in the 24V communication chain between the thermostat's Y (cooling) terminal and the air handler or condenser. When the T9 displays "Cool" and the target temperature is lower than the ambient temperature, the internal relay closes to send power from the R (power) wire to the Y (cooling) wire. If the unit doesn't kick on, the relay may be failing, or more commonly, the C-wire (common) is not providing stable power, causing the thermostat to "ghost" the command without actually triggering the physical relay. Another frequent cause is a tripped float switch in the condensate drain pan. Many modern HVAC systems are wired so that the float switch breaks the R-wire connection to the thermostat specifically to prevent the AC from running while the drain pan is overflowing. In this scenario, the T9 will think it is calling for cooling because its internal logic is active, but no voltage is actually reaching the cooling circuit. You can confirm this by checking if the indoor blower fan is running; if the fan runs but the outdoor compressor doesn't, the issue is likely the contactor or the capacitor. If neither runs, the power loop is interrupted.

How to Fix It

To resolve this, follow these precise steps to isolate the failure point:

1. Force a Manual Call: On the T9 screen, press Menu > System Mode. Ensure it is set to Cool. Now, manually drop the setpoint temperature to 50°F (or at least 5 degrees below the current room temperature). Wait 5 minutes; the T9 has a built-in compressor protection delay to prevent short-cycling.
2. Inspect the Wiring: Pull the T9 faceplate off the wall bracket. Ensure the wire inserted into the Y terminal is stripped to exactly 1/4 inch and is clamped tightly. Tug gently on the wire to ensure it hasn't slipped out of the push-terminal.
3. Check the Condensate Pan: Locate your indoor air handler. Find the PVC drain line and look for a plastic float switch (a small device with a floating puck). If the pan is full of water, the switch is triggered. Use a wet/dry vac to suck out the clog from the exterior drain line until the float drops and the circuit closes.
4. Test the Relay: If you have a multimeter, set it to 24VAC. Place one probe on the R terminal and the other on the Y terminal while the thermostat is calling for cool. You should see approximately 24-28V. If you see voltage here but the AC isn't running, the problem is at the furnace control board or the outdoor contactor.

If That Didn't Work

• Bypass the Thermostat: To determine if the T9 hardware is faulty, perform a jumper test. Turn off the breaker. Remove the T9 faceplate and use a small piece of insulated copper wire to jump the R terminal directly to the Y terminal. Turn the power back on. If the AC kicks on immediately, the T9 relay is dead and the unit needs replacement.
• Check the HVAC Fuse: Go to the indoor air handler and open the panel. Look for a 3-amp or 5-amp automotive-style blade fuse on the control board. If the filament is broken, the thermostat will have power (via a battery or C-wire) but cannot trigger the cooling system. Replace it with an identical amperage fuse.
• Verify the C-Wire Connection: The T9 is power-hungry. If you are using a "C-wire adapter" or a power extender kit instead of a dedicated common wire, the voltage may be dipping too low to engage the Y-relay. Ensure the C-terminal is receiving a steady 24V from the transformer.
• Reset the Control Board: Power cycle the entire HVAC system by flipping the breaker for both the indoor air handler and the outdoor condenser for 60 seconds. This clears any "lockout" codes on the HVAC control board that might be preventing the AC from starting despite the T9's request.

Heads Up

• Compressor Delay: Never rapidly toggle the temperature up and down to test the system; the T9 implements a 5-minute delay to protect the compressor from pressure spikes.
• Filter Maintenance: A severely clogged air filter can cause the evaporator coil to freeze, triggering a limit switch that kills power to the Y-terminal to prevent liquid slugging the compressor.
• Voltage Warning: Always turn off the HVAC breaker before touching the wiring behind the T9 faceplate to avoid blowing the 3A fuse on the control board.
• Battery Backup: The T9 relies heavily on the C-wire. If you notice the screen flickering or the Wi-Fi dropping frequently, your C-wire connection is unstable and will eventually lead to cooling failures.