Honeywell Thermostat Says AC Power Loss – Causes & Fixes
If your Honeywell thermostat says AC power loss, you’re probably dealing with a power issue that needs immediate attention. This error usually means that the thermostat isn’t receiving power from the HVAC system, leading to cooling or heating failures. In this guide, we’ll break down the causes of this problem and how you can fix it.
🔹 What Does “AC Power Loss” Mean on a Honeywell Thermostat?
When your Honeywell thermostat displays “AC Power Loss,” it’s telling you that it has lost its primary power source. Honeywell thermostats typically get power from the HVAC system via a low-voltage transformer.
If there’s an issue with the power supply, the thermostat may temporarily run on battery backup (if applicable) or shut down completely.
🔹 Common Causes of Honeywell Thermostat Says AC Power Loss
Several issues can trigger this error. Below are the most common reasons:
1️⃣ Tripped Circuit Breaker
A power surge or overload can cause the circuit breaker connected to your HVAC system to trip, cutting off power to the thermostat.
2️⃣ Blown HVAC System Fuse
Many HVAC systems have a fuse on the control board. If this fuse blows, the thermostat won’t receive power.
3️⃣ Loose or Damaged Wiring
Loose or disconnected thermostat wires, especially the C-wire (common wire), can cause power loss.
4️⃣ Malfunctioning Transformer
The HVAC system’s transformer converts high-voltage power into the low voltage required for the thermostat. If it fails, the thermostat will lose power.
5️⃣ HVAC System Power Switch Turned Off
Sometimes, the power switch for the furnace or air handler gets turned off accidentally.
6️⃣ Dead Batteries (If Applicable)
If your thermostat uses batteries as a backup, weak or dead batteries may cause it to display a power loss warning.
7️⃣ Dirty or Corroded Wire Connections
Dust, corrosion, or rust on wire terminals can interfere with the electrical connection.
🔹 How to Fix “AC Power Loss” on a Honeywell Thermostat
Now that you know the possible causes, let’s go through the step-by-step solutions.
✅ Step 1: Check and Reset the Circuit Breaker
- Locate your home’s electrical panel.
- Look for any tripped breakers. If the HVAC breaker is in the “OFF” position, switch it back to “ON.”
- Wait a few minutes to see if the thermostat regains power.
✅ Step 2: Inspect and Replace the HVAC Fuse
- Turn off power to your HVAC system.
- Open the furnace or air handler panel and locate the control board.
- Check for a blown fuse (usually a 3- or 5-amp fuse).
- If the fuse is blown, replace it with one of the same rating.
✅ Step 3: Examine Thermostat Wiring
- Remove the thermostat cover.
- Check if the C-wire and other connections are securely in place.
- If you see damaged wires, replace them or call an HVAC technician.
✅ Step 4: Test the HVAC Transformer
- Use a multimeter to check for voltage at the transformer.
- If the transformer is not delivering power, it may need to be replaced.
✅ Step 5: Make Sure the HVAC Power Switch is ON
- Locate the furnace power switch (usually near the furnace or air handler).
- If it’s OFF, turn it back ON and check if the thermostat gets power.
✅ Step 6: Replace the Thermostat Batteries (If Needed)
- Remove the thermostat from its base.
- Replace old batteries with fresh AA or AAA batteries (depending on your model).
- Reattach the thermostat and check if the error disappears.
✅ Step 7: Clean Wire Terminals
- Turn off power to the thermostat.
- Remove any dust or corrosion from the wiring terminals.
- Reconnect the wires securely.
🔹 When to Call a Professional
If you’ve tried all the troubleshooting steps and your Honeywell thermostat still shows “AC Power Loss,” it might be time to call an HVAC technician. You should seek professional help if:
✔️ The circuit breaker keeps tripping repeatedly
✔️ The fuse blows again after replacement
✔️ There’s visible damage to the wiring or transformer
✔️ The HVAC system isn’t responding at all
🔹 Preventing AC Power Loss Issues in the Future
To avoid running into this issue again, follow these maintenance tips:
✔️ Check your HVAC system’s power supply regularly
✔️ Replace thermostat batteries every 6-12 months (if applicable)
✔️ Inspect and clean thermostat wiring during seasonal HVAC checkups
✔️ Ensure the transformer and control board are in good condition
People also ask
What does AC power loss mean in Honeywell?
AC Power Loss on a Honeywell thermostat means that the device has lost its primary power source from the HVAC system.
This could be due to a tripped circuit breaker, blown fuse, faulty wiring, or a malfunctioning transformer.
The thermostat may switch to battery backup (if available) or shut down completely until power is restored.
How do I fix my AC power loss?
To fix AC Power Loss on your Honeywell thermostat:
1️⃣ Check the Circuit Breaker – Reset if tripped.
2️⃣ Inspect the HVAC Fuse – Replace if blown.
3️⃣ Verify Thermostat Wiring – Ensure all connections are secure.
4️⃣ Test the Transformer – Replace if faulty.
5️⃣ Turn On the HVAC Power Switch – Ensure it’s not accidentally off.
6️⃣ Replace Batteries (If Applicable) – Use fresh batteries.
7️⃣ Clean Wire Terminals – Remove dust or corrosion.
Why does my thermostat say no AC power?
Your Honeywell thermostat says “No AC Power” because it’s not receiving power from the HVAC system.
This can be due to a tripped breaker, blown fuse, loose wiring, a faulty transformer, or the HVAC power switch being off.
If your thermostat has batteries, it may run on backup power until the main issue is fixed. ✅
Why did my Honeywell thermostat lose power?
Your Honeywell thermostat lost power due to a tripped circuit breaker, blown fuse, loose or damaged wiring, faulty transformer, or HVAC system power switch being off.
If it runs on batteries, they may be dead. Check these issues to restore power. ✅
Final Thoughts – Honeywell Thermostat Says AC Power Loss
If your Honeywell thermostat says AC Power Loss, it’s usually due to a power supply issue from your HVAC system. Start by checking the circuit breaker, fuse, and thermostat wiring.
If you can’t find the problem, it’s best to consult an HVAC expert. Regular maintenance can also help prevent power-related issues in the future.
