i replaced the batteries but the thermostat still says 'battery low' and won't let me change settings.

Diagnosis

The Honeywell Home RTH9585WF is designed to be powered primarily by the 24VAC transformer in your HVAC system via the "C" (Common) wire. While it accepts two AAA alkaline batteries, these are intended strictly as a backup power source during power outages or as a temporary bridge. When the thermostat displays a persistent "Battery Low" warning despite fresh cells, it indicates a failure in the primary power delivery circuit. The device has detected that it is not receiving the required 24V from the wall, causing it to enter a low-power protection mode that locks out user settings to prevent the batteries from draining instantly. This usually happens for one of three reasons: the C-wire is not connected at the furnace/air handler end, the wire has a break or a loose connection at the terminal block, or the 24V transformer has tripped a fuse. Because the Wi-Fi radio in the RTH9585WF is power-hungry, it will deplete standard AAA batteries in a matter of days if the C-wire isn't providing constant current. You can confirm this by removing the thermostat from the wall plate and using a multimeter to check for 24VAC between the R (Red) and C (Common) wires; if you read 0V or significantly less than 22V, the issue is in your wiring, not the batteries.

How to Fix It

To resolve the power lockout and clear the battery warning, you must ensure the thermostat is receiving constant 24V power. Follow these steps:

1. Shut off the power to your HVAC system at the circuit breaker to avoid blowing the integrated fuse on your control board.
2. Pull the thermostat body straight off the wall plate. Remove the new AAA batteries and set them aside.
3. Inspect the wiring on the wall plate. Ensure the wire in the C terminal is stripped back exactly 1/4 inch, with no frayed copper strands touching adjacent terminals (specifically the R or W terminals).
4. Go to your furnace or air handler. Open the panel to locate the control board. Find the terminal strip and verify that the wire colored to match your thermostat's C-wire is securely fastened to the COM or C terminal. If the wire is tucked back into the wall or not connected at the furnace, the thermostat will never receive power.
5. If you do not have a C-wire, you must install a C-wire adapter kit (such as the Honeywell C-Wire Adapter) or run a new 18/5 thermostat cable.
6. Restore power at the breaker.
7. Snap the thermostat back onto the wall plate. The "Battery Low" message should disappear immediately as the device switches from battery backup to 24V AC power.
8. Navigate to the Menu > Equipment Setup to verify that the device recognizes the power source.

If That Didn't Work

• Check the HVAC Fuse: Locate the purple or amber 3A or 5A blade fuse on the furnace control board. If this fuse is blown, the thermostat will receive no power regardless of the wiring. Replace the fuse with one of the exact same amperage to restore the 24V circuit.
• Test with a Different Power Source: If you suspect the internal transformer is dead, use a 24VAC plug-in transformer. Connect the two wires from the transformer directly to the R and C terminals on the thermostat wall plate to see if the "Battery Low" error clears.
• Inspect for "Power Stealing" Issues: Some older systems attempt to "steal" power from the Rh/Rc line. If your system is doing this, the RTH9585WF may struggle to maintain a stable voltage. Ensure the jumper between R and Rc is properly seated or removed depending on your specific heating/cooling configuration.
• Hard Reset the Logic Board: Remove the batteries and the wall power for 15 minutes. This clears the volatile memory and forces the thermostat to re-poll the power terminals upon reboot, which can sometimes clear a "stuck" battery warning.

Heads Up

• Battery Quality: Only use high-quality Alkaline batteries. Avoid rechargeable NiMH batteries, as they have a lower nominal voltage (1.2V vs 1.5V), which can trigger the "Battery Low" warning prematurely even when fully charged.
• Wi-Fi Drain: If you are operating without a C-wire, the Wi-Fi functionality will drain batteries rapidly. Disable the Wi-Fi in the settings if you cannot install a C-wire to extend battery life.
• Wiring Caution: Never touch the R and C wires together while the power is on; this will instantly blow the fuse on your furnace control board.
• Terminal Tension: Ensure the push-terminals on the wall plate are clicking firmly. A loose wire in the C-terminal is the most common cause of intermittent power loss and "Battery Low" errors.