Replace the battery as soon as possible once the display flashes "Batt Lo" to avoid an unintentional loss of power. The display will begin flashing roughly 30 days before the battery runs out.
Honeywell's programmable thermostats are designed to be easy to use under normal circumstances. And when it is time to troubleshoot, most of the solutions are simple for the owner to execute. If your Honeywell programmable thermostat flashes "Batt Lo," you simply need to replace the battery. If the thermostat is malfunctioning and will not reset, simply cut the power to the thermostat and it will reset itself.
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Troubleshoot "Batt Lo"
Grip the face plate of the thermostat and pull it away from the wall plate to expose the batteries.
Pull out the three AAA batteries.
Put in three fresh AAA batteries. Make sure the positive and negative terminals on the battery are lined up with the positive and negative receptors on the thermostat.
Replace the thermostat's cover. The display will no longer flash "Batt Lo."
Troubleshoot a Honeywell Programmable Thermostat that Won't Reset
Find the circuit breaker in your home that controls the electricity to the programmable thermostat.
Turn the circuit breaker off and keep it off for 5 seconds.
Turn the circuit breaker back on.