Your home thermostat acts as the brain of your heating and cooling system — telling your HVAC when to turn on, turn off, heat, or cool. When it starts to fail, you might notice weird temperature behavior, higher energy bills, or your HVAC running at the wrong times. Catching a bad thermostat early can prevent costly HVAC damage and uncomfortable indoor temperatures.
Is your HVAC acting up? Discover 4 common symptoms of a failing thermostat and how to fix them before they lead to comfort issues or costly repairs.
Your HVAC Doesn’t Turn On or Off Automatically
One of the most common signs your thermostat is failing: your heating or cooling system no longer starts or stops when it should. A bad thermostat may stop sending the correct signals to the HVAC unit, leaving your system silent when you need comfort or running nonstop when you don’t.
What to check:
- Start with checking your circuit breakers to make sure all units are getting the power they need. Even a gas furnace will have electrical components.
- Make sure your thermostat is set to HEAT in winter and COOL in summer.
- Confirm settings aren’t accidentally on “FAN” only.
- Replace batteries if your thermostat uses them. Modern thermostat displays will show a low battery symbol.
FAN setting– you may think your central air system is running hot/cold when it is actually only the fan running. The circulation fan function simply moves air through the house by the intake grills, and out through the registers. There is no heating or cooling at this time. It should turn off after the program has completed. If not, you have a problem to pursue. Turn the system to “off” to eliminate any chance the fan is running.
When the system is set to “AUTO” it will turn off and on by itself, according to the indoor temperature detected at the thermostat. This is not an energy efficient way to moderate your temp in your home. You do not want to switch between heating and cooling frequently.
If the programming seems complicated, you can always look up a user manual online for your specific thermostat.
Where is the Thermostat Located?
First, check the location of your thermostat to make sure it’s not too near a window or a doorway. When a thermostat is located too close to outside air, it can make your system think it’s cooler or warmer than it actually is inside your home. This can also happen if your thermostat is located in a closed off bedroom where the temperature is warmer than the rest of the house.
Contact a reputable HVAC company to help you relocate the device to a better location.
Read Also: Why is My Central AC Running and Not Cooling the House?
The HVAC System will Not Turn On Automatically
The furnace or air conditioning often gets blamed when it is actually a malfunctioning thermostat at fault. After you’ve checked the batteries, you’ll need to determine if there is a fault in the wiring.
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The HVAC System Will Not Turn OFF Automatically
Another sign that your thermostat is bad is when the system won’t turn off.
When this happens it may be that your thermostat is located in a bad area, or it could also signify wiring or programming issues with your system.
If your system runs for long periods of time, it can be hard on the HVAC unit. It will also raise the cost of your electric bill.
Blank or Unresponsive Thermostat Display
A thermostat that won’t turn on, blanks out, flickers, or refuses to respond to button presses is a red flag. This can happen because of dead batteries, power issues, worn-out wiring, or internal thermostat failure.
Symptoms include:
- Display stays dark even with fresh batteries.
- Buttons don’t register inputs.
- Screen resets or shows strange readings.
If simple fixes don’t work, it’s often time to replace the thermostat. A replacement thermostat can be purchased at your local hardware store, or online at retailers such as Amazon starting at around $27 for a simple model. Click here.
You can also get smart thermostats that work with your smart phone and have advanced programming options. Most are easy to install with simple instruction guides.
Note: we may receive a small commission off your purchase at no additional cost to you.
Inaccurate Temperature or Constant Cycling
If your thermostat shows a temperature that doesn’t match the real room temperature, your HVAC will run at the wrong times.
Signs to watch for:
- Set temperature → No change in room temps.
- Unit keeps turning on/off rapidly (short cycling).
- Rooms are unevenly heated or cooled.
These issues can also increase energy bills because your system works harder for less comfort. Again, the remedy is to replace the thermostat.
If your thermostat is new, it may still be covered under factory warranty. Problems occurring electronically with your thermostat would fall under factory warranty. This means it will be replaced at no cost to you. Contact the manufacturer or home builder for details.
Dirt or Wiring Problems in older thermostats
A thermostat doesn’t need heat, cold, or dust near it! Dust buildup is especially problematic for older thermostat models. Loose or corroded wires also cause communication problems between the thermostat and your HVAC system.
Quick checks:
- Clean the interior gently if it’s an older mechanical thermostat.
- Make sure the thermostat isn’t mounted near direct sunlight or drafts.
- Tighten loose wiring connections — or call an HVAC pro if unsure.
How to fix it
Cleaning it is easy, simply open the box and wipe the surfaces down – making sure to get all dust out. Any signs of wiring issues and you’ll need to get a replacement. Older thermostat models cannot be repaired.
Bonus: When to Call a Professional
If you’ve tried simple fixes like changing batteries, adjusting settings, or cleaning the unit and your thermostat still behaves erratically, it’s likely time for a replacement or professional inspection. Faulty thermostats can strain your HVAC, raise energy bills, and lead to bigger repairs if ignored.
Conclusion
Your thermostat might be small, but it has a huge job. Watch for these key warning signs:
✔ HVAC doesn’t start/stop as expected
✔ Blank or unresponsive display
✔ Inaccurate temperature control or short cycling
✔ Dust buildup, wiring issues, or poor placement
Addressing these issues early can keep your home comfortable and reduce costly repairs down the line.
Troubleshooting A Bad Thermostat
Turn the Thermostat Temperature Up or Down by 5 Degrees
Depending on what season you are currently in, you’ll want to complete one of these steps:
- Summer: Set the temperature 5 degrees cooler than you normally would.
- Winter: Set the temperature 5 degrees warmer than you normally would.
When you adjusted the temperature, did you hear a click? If not, wait a few minutes and check if your registers are blowing air out, and if the return vents are sucking air in. No click and no air flow means the HVAC system is not getting the message to turn on.
If the thermostat itself appears to be working correctly and has batteries, etc., then the problem may be with your HVAC unit. Your furnace may be frozen up, in which case you need a professional to repair.
Check Wires and Test Wire Connections
The final step is to remove the cover from the thermostat, and check all wires to ensure they are firmly connected to their mounting screw.
If all wires are properly connected, but the thermostat still doesn’t work you’ll probably need to hire a professional. They will open the device and test each wire individually to determine if any are faulty.
Comment now and let us know what the problem was and how it was fixed.
