my honeywell wifi thermostat screen is just blank, no backlight, nothing. it was working yesterday.

Diagnosis

The Honeywell Home RTH6580WF is not a battery-powered device; it relies entirely on a 24VAC power supply delivered from the HVAC system's transformer via the R (Red) and C (Common) wires. A completely blank screen indicates a total loss of voltage to the thermostat baseplate. This is rarely a failure of the thermostat hardware itself and is almost always a result of a break in the electrical circuit between the furnace/air handler and the wall unit. The most common trigger for this sudden blackout is a tripped safety switch or a blown fuse on the integrated furnace control board. For example, if your AC condensate drain line is clogged, a float switch will trigger, instantly cutting power to the R-wire to prevent your house from flooding. Alternatively, a momentary short circuit in the low-voltage wiring—often caused by a rubbing wire or a pest chewing through insulation—will pop the 3-amp or 5-amp blade fuse on the control board to protect the transformer from burning out. You can confirm this by using a multimeter to test for 24VAC between the R and C terminals on the thermostat baseplate; a reading of 0V confirms a power delivery failure.

How to Fix It

Follow these steps in sequence to restore power to your RTH6580WF:

1. Reset the Circuit Breaker: Go to your main electrical service panel. Locate the breaker labeled "Furnace," "Air Handler," or "HVAC." Even if it looks "ON," flip it completely to "OFF" and then back to "ON" to reset any partial trips.
2. Inspect the Condensate Float Switch: Locate your indoor AC unit. Find the PVC drain line. If there is a plastic switch (a "float switch") installed in the line or the overflow pan, check if the float is lifted. If the pan is full of water, clear the clog using a wet/dry vacuum from the outside drain line; once the water recedes, the switch will reset and power will return.
3. Check the Control Board Fuse: Shut off the power to the HVAC unit. Remove the blower compartment door (usually two knobs or a few screws). Locate the control board—a green circuit board with various wires. Look for a small, colorful automotive-style blade fuse (typically purple for 3A or amber for 5A). Pull the fuse out and hold it up to a light. If the metal filament inside is broken or the glass is charred, replace it with an identical amperage fuse from a hardware store.
4. Verify the Door Switch: Ensure the furnace blower door is fully closed and latched. Most Honeywell-compatible systems have a "door kill switch" that cuts all power to the thermostat if the panel is not seated perfectly flush.
5. Reseat the Thermostat Faceplate: Gently pull the RTH6580WF screen off the wall. Inspect the pins on the back to ensure none are bent. Firmly snap it back onto the baseplate, ensuring it clicks into place to establish a solid connection with the R and C terminals.

If That Didn't Work

• Test the 24V Transformer: Use a multimeter set to AC voltage. Probe the two screws on the transformer (usually located inside the furnace or mounted to the side). If you see 0VAC here, the transformer has failed and requires replacement. If you see 24-28VAC but the thermostat is still blank, there is a break in the wiring between the transformer and the wall.
• Bypass the C-Wire: If you recently installed this model and never had a "C" (Common) wire, the unit may have been running on "power stealing" and finally exhausted its internal capacitor. Check if you have an unused blue or black wire in the wall. Connect that wire to the "C" terminal on both the thermostat and the furnace control board to provide dedicated power.
• Check for a Blown 120V Fuse: Some high-efficiency furnaces have a secondary fuse on the main power entry board before the transformer. Check for any burnt smells or blackened components on the primary power input side of the HVAC system.
• Perform a Hard Reset: Remove the thermostat from the wall and leave it off for 60 seconds. While off, check the wiring for any loose strands touching other terminals, which could be causing a short that immediately blows any new fuse you install.

Heads Up

• Never Oversize the Fuse: If you find a blown 3-amp fuse, do not replace it with a 5-amp or 10-amp fuse. Doing so removes the safety protection and can lead to the entire control board frying, which is a significantly more expensive repair.
• Avoid "Wire Pulling": When reseating the thermostat, be careful not to pull the wires out of the terminal blocks. The RTH6580WF uses push-terminals that can easily loosen, leading to intermittent power loss.
• Seasonal Maintenance: To prevent future blank-screen events, clear your AC condensate drain lines with a vacuum every spring. This prevents the float switch from cutting power during the peak of summer.
• Voltage Range: Ensure your transformer is outputting between 24VAC and 28VAC. Anything over 30VAC can permanently damage the electronic components of the WiFi radio in the thermostat.