Is Your Thermostat Broken? 7 Signs of a Bad Thermostat

Your home’s comfort system rarely fails without any warning. An unresponsive screen, a room that never feels quite right, or a sudden spike in your energy bill are all common signs of a bad thermostat.

Many thermostat issues are simple, such as dead batteries that can be changed in minutes. Others are clear warnings that require a professional

In this guide, I’ll break down what each symptom means and provide insights from HVAC experts so you can decide your next step.

1. The Thermostat is Unresponsive

Potential Cause: Dead batteries, tripped circuit breaker, or tripped safety switch. DIY Solution: Replace the batteries and check the circuit breaker. Call a Pro When: The screen is still blank after trying the simple fixes.

The display might be completely dark, or it may not respond when you press the buttons. This usually points to a power issue. Billy Gregus, Owner of Integrity Refrigeration & AC, suggests a simple reset for DIY troubleshooting.

He advises homeowners to “flip your breaker off for 60 seconds to reset the thermostat’s processor.” If that does not work, the issue could be the backup battery.

*Warning: Gregus warns that if the date and time do not reset after a power cycle, “your backup battery is dead and probably leaking corrosive acid inside the unit. Replace the thermostat immediately because that acid will destroy your wiring next.”

Here’s what to do next:

• Replace the batteries with a fresh set.
• Check your home’s circuit breaker panel. The thermostat is on the same circuit as your indoor furnace or air handler. Find the breaker and flip it completely off, then back on.
• Turn off the power at the breaker and remove the thermostat cover. Look for any loose, frayed, or corroded wires. Gently tighten any loose connections with a screwdriver.

If the screen is still blank after these steps, it is time to call a professional for help.

2. The HVAC System Won’t Turn On or Off

Potential Cause: Faulty wiring, miscalibration, or incorrect fan settings. DIY Solution: Check the thermostat settings. Call a Pro When: The system still runs constantly or won’t turn on after you check the settings.

Another, less obvious, sign of a problem with your thermostat is when your HVAC system doesn’t follow commands. You might adjust the temperature, but the furnace or the air conditioner still won’t start.

The opposite can also happen, where the system runs nonstop even after reaching the right temperature.

Here’s what to do:

• Make sure your thermostat is set to “Heat” in the winter or “Cool” in the summer.
• Confirm the fan is set to “Auto,” not “On,” if you do not want it to run constantly.
• If the system won’t turn off, try switching the thermostat off completely. If it keeps running, you likely have a wiring or calibration issue that needs a professional.

If the system won’t turn on, a professional should inspect it for faulty wiring or internal component failure.

3. Poor Temperature Control

Potential Cause: Poor thermostat location, dirty sensors, or miscalibration. DIY Solution: Clean the thermostat and check its location. Call a Pro When: Your home is still uncomfortable after cleaning the unit and checking for placement issues.

Eli Zimmer, Director of Operations at Luxaire HVAC Services, highlights, “The most common warning sign that a thermostat isn’t working properly is if you are constantly noticing that your home feels warmer or colder than the temperature you’ve set it at. If you are used to a certain temperature, often it’s pretty easy to tell if it’s even a degree or two off.”

This means the thermostat is not reading the room’s temperature accurately. Gregus says uneven heating is a behavior that screams “call a pro immediately.” 

He explains, “When your thermostat works fine, but only certain rooms get cool or warm air. Most people blame ductwork, but I’ve traced this to faulty zone control wiring or thermostat relay failures dozens of times.”

Here’s how to repair your broken thermostat:

• Place a separate room thermometer next to your thermostat for 15 minutes. If the readings are different by more than a degree, the thermostat is inaccurate.
• Check the thermostat’s location. It should be on an interior wall away from direct sunlight, drafts, and heat sources.
• Turn off the power at the breaker, and remove the thermostat cover. Use a soft brush to gently remove dust from the interior components, especially the sensors. Dust on the sensors can insulate them, leading to bad readings.

If these steps do not work, call a professional to recalibrate the unit or move it to a better location.

4. The System is Short Cycling

Potential Cause: Clogged air filter, faulty thermostat, or oversized HVAC unit. DIY Solution: Change the air filter. Call a Pro When: Short cycling continues after you replace the filter.

Short cycling is when your HVAC system turns on and off for very short periods. Gregus calls this the “biggest thermostat red flag.” 

He defines it as “when your system kicks on and off every few minutes instead of running proper 15 to 20 minute cycles. This destroys efficiency and hammers your compressor.”

Gregus offers a go-to homeowner test before calling a pro:

“Set your thermostat 5 degrees lower than the current temperature, then use your phone’s stopwatch to time how long the system runs before shutting off. Anything under 10 minutes means either your thermostat’s calibration is off, your system is oversized, or the sensor’s failing.”

Here’s what to do:

• Check your HVAC system’s air filter. If it is dirty, replace or wash it immediately.
• Inspect the thermostat’s location. If it is too close to a supply vent, it will get a blast of conditioned air and shut off prematurely.
• If a new filter and a good thermostat location do not solve the problem, it is time to call an HVAC technician. The issue could be a refrigerant leak or a more serious electrical problem.

5. Your Energy Bills Suddenly Increase

Potential Cause: A miscalibrated thermostat, constant running, or short cycling. DIY Solution: Review other symptoms to find the root cause. Call a Pro When: You cannot identify the reason for the high bills.

An unexpected spike in your utility bill without a change in your habits can be a sign of a faulty thermostat. When a thermostat malfunctions, it can cause the HVAC system to work harder than it needs to, which drives up energy consumption.

Gregus explains, “I had a customer whose unit was cycling every 90 seconds, and their August electric bill jumped $240 compared to the year before,” he says. “That’s your clearest utility statement warning sign.”

Pro Tip: High bills can sometimes be fixed by changing your habits. Having the common myths about thermostat operation debunked can make a big difference in your energy use.

Here’s what to do:

• Look for other signs of a problem, such as the system running continuously or short-cycling.
• Check for inaccurate temperature readings. A thermostat that is off by just a few degrees can force the system to run much longer than necessary over the course of a month.
• If you have ruled out other causes, have a professional inspect your HVAC system and thermostat to find the source of the inefficiency.

6. The Thermostat Forgets Its Settings

Potential Cause: Failing batteries, power supply issues, or software glitches. DIY Solution: Replace the batteries or check for software updates. Call a Pro When: The thermostat continues to lose its settings.

If you have a programmable or smart thermostat, it should remember your settings. If you constantly have to reprogram it because it reverts to a default state, the thermostat is likely failing.

Zimmer notes that if “you keep having to reset the temperature but it doesn’t stay, that’s a sign that there is an issue too.” 

Here’s what to do:

• If the thermostat uses batteries, replace them. Weak batteries may not have enough power to hold settings in memory.
• For smart thermostats, connectivity can be an issue. Check for a stable Wi-Fi connection and ensure the device’s software is up to date, as bugs can cause programming failures.
• If the problem persists, especially with an older unit, it is a strong sign that the thermostat needs to be replaced.

7. The Thermostat is Old

Potential Cause: Worn-out internal components. Call a Pro When: Your thermostat is over 10 years old and showing signs of failure.

Thermostats do not last forever. The average lifespan is about 10 years. Their components can wear out over time, leading to unreliable performance and inaccurate readings.

Zimmer advises homeowners to “keep that in mind to assess if perhaps yours is nearing the end of its lifespan.”

HVAC experts agree that replacing an over a decade old thermostat is a smart, proactive measure. Newer models are more accurate and offer features that can improve comfort and energy efficiency.

Pro Tip: Consider upgrading to a programmable or smart thermostat. These models can save you money on energy bills by automatically adjusting the temperature based on your schedule. Contact a professional to discuss the best replacement options for your home and HVAC system.

FAQ About Thermostats

Find Thermostat Help

If you’re having issues with your thermostat, examine it carefully and see if you can resolve it yourself. If not, you’ll likely have to hire a professional to repair it for you.

If you need help with your thermostat or other HVAC service, contact your local HVAC professionals. They can service your thermostat, air conditioning, or heater to help keep your indoor air clean and comfortable.

Home Gnome delivers the best home service experience at the click of a button. Book reliable, skilled services in seconds and leave the rest to us.

Read more:

• Why Is Your AC Not Blowing Cold Air?
• Why Your Air Conditioner Is Not Turning On

Main Image: A person uses a screwdriver to repair the internal circuitry of a digital thermostat. Photo Credit: Larry / Adobe Stock