my schedule keeps disappearing or resetting to factory times after a power outage.

Diagnosis

The Honeywell Home T4 Pro is designed with a dual-power architecture. While it primarily draws power from the 24VAC transformer via the HVAC system, it relies on a set of backup batteries to maintain the internal clock and the volatile memory where your 5-2 or 5-1-1 schedules are stored. When a power outage occurs, the device switches to battery power to keep the "brain" alive. If those batteries are depleted, missing, or have leaked, the memory chip loses its charge, causing the device to revert to its factory default state—usually resetting the time to 12:00 and wiping all setpoints. You can confirm this is the issue if you notice the screen remains blank during a power outage, or if you see a "low battery" icon flashing on the display shortly before the reset occurs. Even if the screen is currently lit, it may be receiving enough voltage from the C-wire to operate the display, but not enough to bridge the gap during a millisecond power flicker or a total blackout, leading to a total memory wipe.

How to Fix It

To permanently resolve the memory loss and ensure your schedule persists through power fluctuations, follow these precise steps:

1. Power Down the System: Go to your circuit breaker panel and switch off the breaker labeled "Furnace," "Air Handler," or "HVAC." This prevents accidental shorting of the 24V wires while the faceplate is removed.
2. Remove the Faceplate: Grasp the outer edges of the T4 Pro thermostat and pull it firmly and straight away from the wall mounting plate. There are no screws holding the faceplate on; it is a snap-fit connection.
3. Access the Battery Compartment: Locate the battery slot on the rear of the device. Remove the existing batteries. If you see any white powdery residue (acid leakage), clean the contacts with a cotton swab dipped in white vinegar before proceeding.
4. Install High-Quality Batteries: Insert two brand new AAA alkaline batteries. Avoid using rechargeable NiMH batteries, as they have a lower nominal voltage (1.2V vs 1.5V) which can cause the T4 Pro to trigger "low battery" warnings prematurely.
5. Mount and Power Up: Align the pins on the back of the thermostat with the terminals on the wall plate and press firmly until it clicks into place. Restore power at the circuit breaker.
6. Re-establish Schedule: Press the "Menu" button, navigate to "Set Schedule," and enter your desired temperature setpoints for the Wake, Leave, Return, and Sleep periods. Once finished, press "Done" to save the settings to the non-volatile memory.

If That Didn't Work

• Check the C-Wire Connection: If the batteries drain rapidly (within weeks), your thermostat may not be receiving constant power from the HVAC system. Remove the faceplate and ensure the blue or black "C" wire is securely fastened in the C terminal. If the wire is loose or missing, the thermostat relies solely on batteries, which will deplete quickly.
• Inspect the Transformer: If the device resets even with new batteries and a connected C-wire, your 24V transformer may be fluctuating. Use a multimeter to test the voltage between the R and C wires; it should read between 22VAC and 28VAC. If it reads lower, the memory may not be getting a steady charge.
• Perform a Hard Factory Reset: Sometimes a corrupted memory chip causes the schedule to "ghost" or disappear. Navigate to the advanced menu, find the "Reset" option, and confirm. This clears the internal cache and allows you to start the programming from a clean slate.
• Verify Wiring Polarities: Ensure the R (power) wire is not touching any other wires or the metal of the wall plate. A momentary short-circuit can trip the internal fuse or cause a logic reset that mimics a power outage.

Heads Up

• Battery Cycle: Replace the AAA batteries every 12 months regardless of whether the screen looks functional. The T4 Pro does not always provide a clear "Battery Low" warning before the memory wipes.
• Avoid Zinc-Carbon: Only use Alkaline or Lithium AAA batteries. Zinc-carbon batteries have a significantly shorter shelf life and will fail during the first winter cold snap.
• Programming Tip: If you frequently change your schedule, consider using the "Temporary Hold" feature rather than overriding the main schedule, as this prevents accidental permanent changes to your programmed setpoints.
• Static Warning: When handling the back of the thermostat, touch a grounded metal object first to discharge static electricity, as the memory pins are sensitive to ESD (Electrostatic Discharge).