E99
Honeywell Home
Internal Firmware or Hardware Failure
Thermostat displays E99 and is unresponsive or stuck
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Corrupted firmware, Failed internal memory, Severe power surge damage, Defective main PCB
How to Fix / Troubleshooting
Safety: Turn off power to the HVAC system at the breaker before removing the thermostat.
- Power reset: Remove the thermostat from the wall plate and turn off the HVAC breaker. Wait 2 minutes, then restore power and reinstall the thermostat.
- Factory reset: If the thermostat responds to touch, navigate to the advanced settings and perform a factory reset. Reconfigure system type and Wi‑Fi.
- Check for physical damage: Inspect the thermostat for burn marks, cracks, or signs of a surge. If present, the internal electronics are likely damaged.
- Surge protection: If a recent storm or power event occurred, consider installing a surge protector on the HVAC circuit to prevent future failures.
- Replace thermostat: Persistent E99 after reset and power cycling indicates a failed main board. Replace the Honeywell Home smart thermostat.
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Repair Difficulty
Medium
3/5
Required Part
Honeywell Home Smart Thermostat Main Assembly
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