my honeywell wifi thermostat screen is blank and won't turn on, checked the breaker and it's fine.

Diagnosis

The Honeywell Home RTH9585WF is a powered device that requires a constant 24VAC stream to maintain its WiFi connection and illuminate the backlit LCD. Unlike basic battery-operated models, this unit relies heavily on the "Common" (C) wire to complete the electrical circuit. When the screen is completely blank and the breaker is confirmed on, the root cause is almost always a break in the 24V power loop between the HVAC control board and the thermostat wall plate. This failure typically occurs due to one of three reasons: a loose wire nut or terminal screw at either end of the C-wire, a tripped float switch in your AC drain pan (which kills power to the R-terminal to prevent flooding), or a blown 3-amp or 5-amp fuse located directly on the furnace's integrated control board. To confirm, use a multimeter to test for 24V AC between the R (Red) and C (Common) wires at the wall plate; if you read 0V, the issue is upstream in the HVAC system, not the thermostat hardware itself.

How to Fix It

1. Shut off power to the HVAC system at the main breaker panel and the local service switch (usually a toggle switch located on the side of the furnace).
2. Gently pull the RTH9585WF faceplate straight off the wall mounting plate. Note the wire positions before moving anything.
3. Inspect the C-terminal. Ensure the wire (typically blue or brown) is stripped to exactly 1/4 inch and is clamped firmly under the terminal screw. Give the wire a light tug to ensure it doesn't slide out.
4. Go to the furnace or air handler. Open the blower compartment and locate the control board. Find the terminal strip where the thermostat wires connect.
5. Verify that the wire connected to the "C" terminal at the thermostat is also connected to the "C" or "COM" terminal on the furnace board. If the wire is tucked back into the wall and not connected at the board, the thermostat will not power on.
6. If the wire is connected, check the furnace control board for a small automotive-style blade fuse (usually 3A or 5A). If the filament is broken, replace it with an identical amp-rated fuse.
7. Restore power at the breaker and service switch. The screen should initialize and boot to the setup menu within 30 seconds.

If That Didn't Work

• Check the Condensate Overflow Switch. If your AC drain line is clogged, the float switch will trigger and physically break the Red (R) wire connection to prevent water damage. Locate the PVC drain line near the indoor unit; if the trap is full of water or the switch is lifted, clear the blockage with a vacuum or compressed air to restore power.
• Install a 24V C-Wire Adapter. If your home was wired for an older thermostat and lacks a physical C-wire, you cannot simply "jump" wires. You must install a C-wire transformer or an add-a-wire adapter kit between the furnace and the thermostat to provide the necessary return path for electricity.
• Test for a Short Circuit. Use a multimeter to check for continuity between the R and C wires while they are disconnected from the thermostat. If there is a direct short, the furnace fuse will blow instantly every time you replace it, indicating a pinched wire in the wall or a faulty solenoid valve in the HVAC system.
• Perform a Hard Reset. While the unit is powered, press and hold the reset button (if accessible via the menu) or disconnect the power for 60 seconds and reconnect. In rare cases, the internal firmware can hang, causing a "black screen" state despite having power.

Heads Up

• Never replace a 3-amp fuse with a 10-amp or 20-amp fuse. Doing so removes the safety mechanism and can lead to the permanent frying of the HVAC control board, which is a costly replacement.
• Avoid over-tightening the terminal screws on the RTH9585WF wall plate. Over-torquing can strip the plastic housing or shear the thin 18-gauge thermostat wire.
• If you are using a "DIY" plug-in 24V transformer instead of the furnace power, ensure you have disconnected the R-wire from the furnace to avoid "back-feeding" two different power sources into the thermostat, which will destroy the circuitry.
• Periodically check your AC condensate line for sludge buildup; a clean line prevents the float switch from tripping and unexpectedly killing your thermostat power during a heatwave.