Honeywell thermostat says AC power loss due to a fault with the HVAC equipment or the power income with the device.
In this guide, we will unwrap all possible causes and of course, provide easy-to-follow solutions and non-technical steps to follow. Let’s keep reading!
When your Honeywell thermostat has the “AC Power Loss” error message, it means that one or more of the ACs in your system are disconnected from power.
The message could also indicate a problem with your thermostat or the HVAC system’s furnace/breaker.
Let’s transcript the meaning behind your Honeywell thermostat error!
What Does AC Power Loss Mean On Thermostat?
“AC Power Loss” on any thermostat usually means that your HVAC system is not receiving enough power from the AC’s electrical outlet.
If an AC installed in your system is plugged into power but doesn’t work due to an electrical fluctuation, the thermostat displays this error.
“AC Power Loss” On Honeywell Thermostat
The AC indicator on the display of a Honeywell thermostat may say “AC Power Loss” when one of the connected AC devices does not get enough power.
As it is for any thermostat, this error should be acted against it as soon as possible to prevent causing more power-related issues!
Causes For “AC Power Loss” On Honeywell?
- The electrical outlet of your AC is faulty
- There’s a temporal AC power outage
- One of your AC’s power is not working
- Your AC cannot withdraw enough power
- General problems with the HVAC system
Note: In rare cases, this problem could also be related to the thermostat and the way you’ve installed the system’s wires so continue reading the guide.
Honeywell Thermostat Says AC Power Loss – Quick Solutions!
To make sure our solutions are as effective as possible, follow our steps in consecutive order and apply each solution only while your Honeywell thermostat is running.
Let’s jump right in!
1. Thoroughly Examine the AC Unit
For our first solution, we will check exactly what the errors say and verify the condition of your AC.
On more complex HVAC systems that have more than one AC, this step may take a while as you must check multiple things with each of your AC units.
How to Inspect an AC Unit’s Power?
We will primarily be inspecting the power supply of your AC and determine if the electricity is constant.
The “AC power loss “error may occur immediately after your AC goes out because its power has been stopped, interrupted, or has experienced a failure.
Do the following:
- Verify the AC is connected to an electrical outlet
- Check the condition of the AC device’s power adapter
- Reconnect the AC’s power adapter from the electrical outlet
- Check if a “Low Power Mode” is enabled on your AC
Note: Incompatible “Low Power” regime of your thermostat may deceive the HVAC system that the device is not receiving enough power and should be turned off!
2. Inspect the Thermostat’s Wiring
The wiring should be checked early, as it could be the cause of the error message you’re seeing on the thermostat’ screen.
There’s a specific way that the cables of your thermostat and the HVAC system should be connected for the system to work properly.
- To reach the wiring, remove your Honeywell thermostat from the wall stand and pull out all of the cables in the wall.
Important: Types of Thermostat Wiring!
There are two sections of cables in the setup of your Honeywell thermostat.
Half of the cables go into the AC Connections unit and the other half into the Furnace connections unit.
| Furnace Connections | Air Conditioner Connections |
| “Rн” to “R” red wire | “G” to “G” green wire |
| “Rc” to “R” red wire | “Y1” to “Y1” yellow wire |
| “W1” to “W1” gray wire | Optional “PEK” and “OB” wires |
Note: All ports and connectors on your thermostat are labeled for easier orientation!
How to Properly Connect Thermostat Cables?
- Red “Rн” furnace cable to “R” port in the furnace connections
- Gray “W1” furnace cable to “W1” in the furnace connections
- Red “RC” furnace cable to “R” on the supplying unit
- Green “G” AC cable, to “G” on the fan relay connector
- Yellow “Y1” AC cable to “Y1” on the compressor’s first stage
Tip: It’s recommended for a first-time installation to acquire help from qualified professionals!
3. Connect your AC to a Different Outlet
Your Honeywell thermostat’s error message may also indicate that your AC is connected to an outlet, but there is no power in that outlet.
The only way to correct this error is to plug the problematic AC into a different electrical outlet that is proven to work.
- Unplug your AC’s power adapter and connect it to a different available outlet
How to Test a Power Outlet?
The easiest way to check if there’s electricity in a wall outlet is by plugging your phone’s charger and your phone there.
If the battery of your phone is filling up, it means that the outlet works!
Note: Preferably, test an outlet from a different room, hence on a different circuit fuse!
4. Power Cycle the Honeywell Thermostat
If the problem still occurs on your Honeywell thermostat, then the next thing to do is the “power cycle” process.
Since your thermostat is battery-operated, any temporal interface can be reset by removing the batteries from the compartment and waiting for several minutes.
Perform a power cycle on your Honeywell thermostat in the following way:
- Power off your Honeywell thermostat device.
- Take the thermostat off the wall and stand next.
- Undo the screws to open the battery compartment.
- Eject the batteries from the thermostat and wait.
- Once 40 seconds have passed, place the batteries.
Alert: Putting the batteries in the wrong way (with negative to positive and positive to negative) will cause the thermostat to reset (learn how in solution #7)
5. Toggle the Safety Switch
To temporarily put your HVAC system into lockdown and reset any faults, you must toggle the safety switch.
The switch prevents your system from working temporarily and is used during flood situations or fatal system errors.
- If you have a safety switch, toggle it in the furnace room for 15 seconds!
Note: Once 15 seconds have passed, make sure to put the safety switch towards “OFF”.
6. Check the Fuse/Circuit Breaker
There’s a fuse in the circuit breaker responsible for delivering power to your HVAC system and the thermostat.
If the fuse has been blown or someone has accidentally tripped the circuit breaker, your thermostat may display “AC power loss” after booting up.
- To fix powering issues with the HVAC system, you must reset the circuit breaker!
How to Reset Honeywell Circuit Breaker?
Acquire access to the circuit breaker panel and locate the fuse labeled with “Thermostat” or “HVAC”.
Once you’ve found it, push the lever down and wait for 10 seconds to do a reset.
How to Check for Blown HVAC Fuses?
In the very same circuit breaker panel, locate the fuse that has the “HVAC” or “Thermostat” label.
Check if the fuse’s top part seems like it opened or has been burned up (looks black).
Note: To replace a blown fuse, unscrew the current one from the circuit breaker and take it to a nearby technical shop to purchase an exact replacement.
7. Factory Reset the Honeywell Thermostat
Perhaps, resetting your Honeywell thermostat may be the only solution that would work against the “AC Power Loss” error message.
To reset your Honeywell, all you have to do is insert the batteries of the device with negative facing positive and positive facing negative.
Alert: The factory reset will permanently erase any thermostat settings and configuration!
Let’s learn how to perform a reset in more detailed steps:
- Take your Honeywell thermostat off the wall.
- Remove the battery compartment cover.
- Eject the batteries inside the thermostat.
- Place the batteries with “+” facing “–” and “–” facing “+”.
- In other words, install the batteries incorrectly!
- Wait for 15-20 seconds until the thermostat resets.
- Place the batteries with “+” facing “+” and “–” facing “–”.
Info: Your thermostat may take up to 3 minutes to startup once the factory reset has been completed so remain patient.
Quick Recap:
A quick solution to the Honeywell thermostat says AC power loss problem is to reset the circuit breaker and then power cycle the thermostat. If the issue persists, thoroughly inspect the problematic AC and perform a factory reset on your Honeywell thermostat.
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Finn Wheatley holds a Master’s Degree in Computer Science from UCL, London. He helped small data science consultancy firms, helping large corporations to grow their data capabilities, deploy advanced machine learning-based analytics and troubleshoot tech-related issues. Check out more about him here.