getting an "Err" message on the screen, it flashes and the system won't run at all.

Diagnosis

The "Err" message on a Honeywell Home RTH9585WF is a general system fault indicator, typically triggered when the internal logic board detects a critical mismatch between the configured equipment settings and the electrical signals returning from the HVAC system. This often happens after a power surge or a brownout, which can corrupt the volatile memory of the thermostat, causing it to "forget" whether it is controlling a heat pump, a conventional furnace, or an electric heat strip. Specifically, this error occurs when the thermostat attempts to engage a relay (like the Y or W wires) but detects an open circuit or an unexpected voltage spike that doesn't align with the programmed equipment profile. If the screen is flashing, it indicates a boot-loop or a failure to complete the Power-On Self-Test (POST). You can confirm this is a configuration or power issue if the display remains unresponsive to temperature adjustments and the HVAC system fails to click on even when the set point is raised significantly.

How to Fix It

To resolve the "Err" state, you must perform a hard power cycle and a full reconfiguration of the Installer Setup (ISUP) menu. Follow these exact steps:

1. Locate your circuit breaker panel and switch off the breaker labeled "Furnace," "Air Handler," or "HVAC." Wait exactly 60 seconds to allow the capacitors on the RTH9585WF control board to discharge completely.
2. Gently pull the thermostat body straight off the wall plate. Inspect the wires—specifically the R, C, W, and Y terminals. Ensure no copper strands are touching adjacent terminals and that each wire is inserted fully into the spring-loaded connector.
3. Snap the thermostat back onto the wall plate and restore power at the breaker.
4. If the "Err" message persists, enter the Installer Setup menu by pressing and holding the Menu button for approximately 5 seconds until the screen changes.
5. Use the arrow keys to navigate to the ISUP (Installer Setup) section.
6. Navigate to the equipment configuration settings. You must verify the following specific codes:
   • Heating System: Ensure it is set to 1 (Gas/Oil) or 2 (Electric) based on your actual unit.
   • Cooling System: Ensure it is set to 1 (Electric) for standard AC.
   • Heat Pump: If you have a heat pump, ensure this is set to "Yes" and verify the O/B reversing valve setting (usually O for most brands like Carrier/Trane).
7. Press Done or Save to commit the settings. The thermostat will reboot, and the "Err" message should clear.

If That Didn't Work

• Check the C-Wire Voltage: Use a multimeter to measure the voltage between the R (Red) and C (Common) wires. You should see a steady 24VAC. If the voltage is fluctuating or below 22VAC, the thermostat will trigger an "Err" message due to under-voltage. This usually indicates a failing transformer in the furnace or a loose connection at the air handler.
• Perform a Factory Reset: If the configuration wizard doesn't clear the error, you may have a corrupted firmware state. Remove the thermostat from the wall, remove any backup batteries if present, and hold the reset button (if accessible) or simply leave it unpowered for 10 minutes. Upon reboot, treat it as a brand-new installation.
• Inspect the HVAC Fuse: Check the 3-amp or 5-amp blade fuse located on the furnace control board. If this fuse is blown, the thermostat will not receive the necessary return signal from the equipment, which can trigger a system error on the display.
• Test for a Short Circuit: Turn off power and disconnect the thermostat. Use a jumper wire to momentarily bridge the R and W terminals at the wall plate. If the heat kicks on, the issue is internal to the RTH9585WF hardware and the unit may need replacement.

Heads Up

• Avoid Over-tightening: When re-seating wires, do not over-tighten the terminal screws or force thick-gauge wire into the slots, as this can crack the solder joints on the wall plate.
• C-Wire Necessity: The RTH9585WF is a WiFi-enabled device and requires a dedicated Common (C) wire for stable operation. Using a "power extender kit" or relying on power-stealing can lead to intermittent "Err" messages during high-demand cycles.
• Firmware Updates: Once the error is cleared and you are back online, check the Honeywell Home app for any pending firmware updates. These updates often include patches for "false positive" system errors.
• Voltage Spikes: If you live in an area with frequent power surges, consider installing a 24V surge protector at the furnace to prevent the RTH9585WF logic board from frying.