thermostat says "Equipment Shut Off" and won't let me change the temperature or run the system.

Diagnosis

The "Equipment Shut Off" message on the Honeywell Home RTH9585WF is not a thermostat failure, but a protective lockout state. The thermostat has detected that the HVAC system is cycling on and off too rapidly (short-cycling) or has failed to reach the set point after multiple attempts. To protect your compressor or heat exchanger from permanent damage, the thermostat enters a hard lockout mode, severing the connection to the equipment. This is typically triggered by a "Limit Switch" trip. In heating mode, this happens when the heat exchanger overheats because airflow is restricted. In cooling mode, it is often caused by a clogged condensate drain line; when the float switch in the drain pan rises, it breaks the R-wire (24V) connection, signaling the thermostat to shut down the equipment to prevent a flood. You can confirm this by checking if your furnace/air handler is blinking a specific LED error code on the internal control board or if there is standing water in the secondary drain pan.

How to Fix It

To clear the lockout and restore functionality, you must first resolve the physical cause of the shutdown before resetting the software.

1. Inspect the Air Filter: Remove your current furnace/AC filter. If it is grey or clogged with dust, replace it immediately. A restricted filter causes the heat exchanger to overheat, triggering the "Equipment Shut Off" message.
2. Clear the Condensate Line: Locate the PVC drain line coming off your indoor evaporator coil. If you see water in the emergency overflow pan, use a wet/dry vacuum to suck out the sludge and algae from the exterior exit point of the PVC pipe.
3. Power Cycle the System: Turn off the HVAC breaker in your main electrical panel for 60 seconds, then flip it back on. This clears the "hard fault" memory on the furnace control board.
4. Perform the Thermostat Reset: Return to the RTH9585WF interface. Press and hold the Fan button for exactly 5 seconds. You will see the screen flicker or refresh, and the "Equipment Shut Off" message should vanish, returning you to the main temperature display.
5. Test the Cycle: Lower the set point (for cooling) or raise it (for heating) by 3 degrees to ensure the system engages and stays running for at least 5 minutes without tripping again.

If That Didn't Work

• Check the Flame Sensor: If the system shuts off again immediately after the reset during a heating cycle, the flame sensor is likely carbon-fouled. This prevents the board from detecting a flame, causing a "hard lockout." You will need to clean the thin metal rod (sensor) with a piece of fine steel wool or a scotch-brite pad.
• Verify the C-Wire Connection: Ensure the common wire (C) is securely fastened to the terminal. If the thermostat is losing power momentarily due to a loose wire, it can misinterpret the power drop as an equipment failure and trigger the lockout. Tighten the screw terminals on the backplate.
• Inspect the Float Switch: If you have a "Safe-T-Switch" or similar float switch installed in your drain line, ensure the float is not stuck in the "up" position. Even if the pan is dry, a stuck switch will keep the R-wire open, maintaining the "Equipment Shut Off" status.
• Reset via Advanced Menu: If the Fan button reset fails, enter the Installer Setup menu by pressing and holding the Menu button for 5 seconds. Navigate to the "Equipment" settings and toggle the system type (e.g., from Gas to Electric and back) to force a software reboot of the equipment logic.

Heads Up

• Filter Frequency: Change your filters every 30-90 days. High-MERV filters (11+) restrict airflow more than basic pleated filters and are the leading cause of "Equipment Shut Off" errors in high-efficiency systems.
• Drain Maintenance: Pour one cup of distilled white vinegar down your condensate drain line every six months to prevent algae buildup and "slime" that leads to float switch trips.
• Voltage Warning: Never touch the control board with a screwdriver while the power is on; a short circuit between the 24V transformer and the board can fry the RTH9585WF's internal circuitry.
• Short-Cycle Protection: If you manually turn the system off and on rapidly during testing, the thermostat may trigger a 5-minute delay. This is normal behavior and not a fault.