5 Signs of a Malfunctioning Thermostat & How to Fix It

Updated: 7/25/25

When your air conditioner or furnace starts acting up, you might assume the problem lies with the big, complex unit itself. But often, the real culprit is the small, quiet controller on your wall: the thermostat. As the brain of your entire HVAC system, even a small malfunction in your thermostat can lead to a home that’s too hot or too cold, high energy bills, and unnecessary wear and tear on your equipment.

If you suspect your thermostat is failing, you've come to the right place. Here are the top five signs of a bad thermostat and the steps you can take to solve the problem.

Top 5 Signs of a Bad Thermostat

Look for these common symptoms to determine if your thermostat is the source of your HVAC troubles.

1. Your AC or Heater Won't Turn On (or Won't Turn Off)

The most obvious sign of a problem is when your HVAC system ignores the thermostat completely. You might find that your AC won't kick on no matter how low you set the temperature, or it runs constantly without ever reaching the set point and shutting off. This often points to faulty wiring or a broken sensor within the thermostat.

2. The Thermostat Display is Blank or Unresponsive

If your digital thermostat display is blank or won't respond when you press the buttons, it's a clear red flag. This could be something as simple as dead batteries, but it could also indicate a tripped breaker or a loss of power from your HVAC system, which points to a deeper wiring issue.

3. Your Home's Temperature Doesn't Match the Setting

Do you feel like your home is much warmer or cooler than the temperature shown on the thermostat? This indicates that the thermostat's temperature sensor is failing. It’s misreading the room's temperature and therefore sending incorrect signals to your HVAC system, causing it to run too much or not enough and wasting significant energy.

4. The AC System "Short Cycles" Rapidly

Short cycling is when your air conditioner turns on and off in rapid, frequent bursts—often every few minutes. While other issues can cause this, a faulty thermostat is a common culprit. Incorrect calibration or bad wiring can send erratic signals to the compressor, causing it to shut down prematurely. This puts extreme strain on your AC and can lead to a costly breakdown.

5. Wi-Fi or App Issues with Your Smart Thermostat

Smart thermostats offer amazing convenience, but they also come with their own unique problems. If your thermostat frequently disconnects from Wi-Fi, won't sync with the smartphone app, or ignores your programmed schedules, the issue likely lies within the device’s software or hardware.

Troubleshooting: What You Can Do Before Calling for Service

Before scheduling a repair, try these simple steps. You might be able to solve the problem in minutes.

Check the Settings: Double-check that your thermostat is on the correct setting ("COOL" for summer, "HEAT" for winter) and not just "FAN."
Replace the Batteries: If your display is blank or glitchy, the first step is always to install a fresh set of high-quality batteries.
Check the Circuit Breaker: Look for a tripped breaker in your home's main electrical panel that is labeled for your HVAC or furnace.
Clean the Thermostat: Gently dust the thermostat's cover and interior with a soft brush to remove any dust buildup that could be interfering with its sensors.

Still having trouble? If you've tried these steps and your system isn't working correctly, it's time for a professional diagnosis.

When to Call a Professional for Thermostat Repair

While the simple fixes above can solve many issues, some problems require an expert. For your safety and to avoid causing more damage to your HVAC system, you should call a NATE-certified technician if you suspect:

Faulty Wiring or Internal Components
A Broken or Inaccurate Temperature Sensor
Complex Smart Thermostat Configuration Issues
A Problem That Is Actually with Your AC or Furnace Unit

Our team has the advanced diagnostic tools to pinpoint the exact cause of the problem, whether it's the thermostat or another part of your HVAC system.