honeywell t9 thermostat display is on but it won't change the temperature when i press the arrows

Diagnosis

The Honeywell Home T9 (RCHT9610WFSW2003) utilizes a software-level safety lock called "Temperature Setpoint Limits." When the display remains active and responsive to touch but the temperature value refuses to budge despite pressing the arrows, the device has reached a hard boundary defined in the Advanced Setup. This is designed to prevent homeowners from accidentally setting the heat to 100°F or the AC to 50°F, which could damage HVAC equipment or cause astronomical energy bills. Another common cause for this specific behavior is the "Hold" function combined with a restrictive schedule. If the thermostat is locked into a specific schedule profile and the user is attempting to override it while a "Permanent Hold" is active with restricted boundaries, the interface may appear frozen. You can confirm this is a limit issue if the arrow flashes briefly but the number does not change, or if the temperature stops exactly at a round number like 90°F or 50°F.

How to Fix It

To resolve this, you must access the Installer Setup menu to expand the allowable temperature range. Follow these exact steps:

1. Press the Menu button on the main screen.
2. Scroll down and select Advanced Setup.
3. Navigate to ISU (Installer Setup). You may be prompted for a code; if so, use the default factory code.
4. Scroll through the ISU numbers until you reach ISU 110 (Heat Setpoint Max). Use the arrows to increase this value (e.g., from 90°F to 95°F) if you are unable to raise your heat.
5. Scroll to ISU 111 (Heat Setpoint Min) and ensure it is set to a reasonable minimum, such as 40°F, to prevent pipe freezes.
6. Scroll to ISU 112 (Cool Setpoint Max) and increase this value if you cannot raise the AC temperature high enough.
7. Scroll to ISU 113 (Cool Setpoint Min) and adjust this if you cannot lower the AC temperature (e.g., changing 70°F to 60°F).
8. Press Done or Exit to save the changes and return to the home screen.
9. Test the arrows again to ensure the temperature now increments and decrements past the previous sticking point.

If That Didn't Work

• Check for Screen Calibration Issues: The T9 uses a capacitive touch screen. If the arrows aren't registering, a buildup of oils or dust can create "dead zones." Clean the screen with a microfiber cloth and a small amount of isopropyl alcohol. If the screen is physically cracked or delaminating, the touch input for the arrows may be permanently disabled.
• Perform a Hard Power Cycle: Remove the T9 faceplate from the wall bracket. Wait exactly 30 seconds to allow the internal capacitors to discharge. Snap the faceplate back onto the wall base. This forces a reboot of the firmware and clears any temporary software glitches causing the UI to freeze.
• Disable "Lock" Mode: Check if the screen lock is enabled. If a padlock icon appears on the display, the arrows are disabled by design. Press and hold the Menu button for 5 seconds or enter the unlock code to regain control of the temperature settings.
• Verify C-Wire Voltage: If the display is on but the logic board is underpowered, the processor may lag or ignore inputs. Use a multimeter to check the voltage between the R and C wires at the backplate; it should read between 24V and 28V AC. If it is dipping below 22V, the thermostat may behave erratically.

Heads Up

• Avoid Extreme Setpoints: While you can increase the limits in ISU settings, setting the heat above 90°F for extended periods can cause some furnace heat exchangers to overheat and trip a limit switch, shutting down your system.
• Firmware Updates: Ensure your T9 is connected to 2.4GHz Wi-Fi. Honeywell pushes "silent" updates that often fix UI responsiveness issues. If the device is offline, it cannot receive these patches.
• Avoid Direct Sunlight: If the T9 is installed in direct sunlight, the internal temperature sensor can overheat, causing the device to enter a protection mode that may limit your ability to adjust settings.