How to Tell if Your Home Thermostat is Bad

5 Signs of a Bad Thermostat

1. It’s Not Responding to Temperature Changes

This could be due to proximity to sources of heat and humidity — such as the thermostat being located on a wall that faces the exterior of your home or being too close to a room with significant temperature fluctuations, such as a bathroom, kitchen, or laundry room. Ideally, the thermostat should be installed on an interior wall, close to where you and/or your family spend the majority of the time (such as near the bedrooms or living room). An HVAC technician can make this change.

2. The AC Keeps Running Constantly

Check to see if the fan is set to on. If this is the case, the AC will continue to run even if it has reached your desired thermostat setting. While the cooling component will be shut off, the fan will continue circulating air. If the switch is set to on, change the setting to auto. If the problem is resolved with this simple fix, the thermostat is working fine. If, however, the AC continues to run without ever cycling off, the relays may be stuck together.

Relays are AC parts that open and close electrical circuits to different parts of your air conditioner. They do this automatically. However, regular wear and tear can cause them to become stuck together — preventing the thermostat from communicating with other parts of the AC. So even if you turn off the thermostat, the system will continue to run. This can only be fixed by a licensed HVAC technician.

3. The AC Won’t Turn On

A common reason for the AC not turning on is a tripped circuit breaker. Set the thermostat to off, switch the breaker to the middle setting, then to on. Then, turn the thermostat back on. If this is not the issue, turn off the thermostat again, remove the cover, and use canned compressed air to eliminate accumulated dust and grime.

Once everything is clean, inspect the wiring to see if any of them are loose. Also, check the screws to see if any of them need to be tightened. Check the inside of the thermostat for any signs of corrosion. If there is, you’ll have to replace the thermostat. If there’s no corrosion and all components are clean and tightened, place a level above the thermostat to see if it’s been slightly moved by an accidental bump. Doing so may throw off the settings and cause the thermostat to malfunction.

4. No LED Display

There are several reasons why a thermostat’s LED display would be blank. The easier ones to resolve include an LCD brightness display that’s too low, dead batteries, or too much dirt inside the thermostat. You can troubleshoot all of these issues by reviewing the AC system’s manual. If your thermostat requires a battery, it’s good practice to always have backups, in case they run out at an inconvenient time — such as the middle of the night. If the issue is dirt inside the thermostat, you can remove the cover and clean the inside with canned compressed air.

If none of the above-mentioned solutions resolve the problem, you may have a tripped circuit breaker, a blown fuse, or a tripped float switch. Check your breaker box for the switch labeled AC. If it’s not on the on setting, switch it to the middle setting before turning it back on. To check if the issue is a blown fuse, walk outside to the condenser unit. Next to it — on the wall — there should be a small metal box. Lift the lid, take out the fuses, and test the voltage with a multimeter. If the fuse is dead, take a picture of the make and model to a hardware store to buy a replacement.

If neither of these is the problem, check the float switch. Air conditioners work by absorbing warm and humid air from inside your home, then removing the moisture from it. This condensation falls into an AC component called a drip pan. As the water level rises, it causes another component — called the float switch — to rise along with it. Once the drip pan is full, the water is siphoned out of your home through a drain line. If the drain line is clogged with mildew and sludge, the drip pan will remain full at all times. At this point, the float switch will become activated and your AC will shut off. To prevent this from happening, clean the drain line regularly.

5. The Thermostat is Old

Thermostats have a long lifespan. Typically, they should last as long as your air conditioner — which could be anywhere between 15 and 20 years, depending on how often you’ve been providing maintenance checks. If your HVAC system is approaching that timeframe, it may be time to upgrade it. This will ensure energy efficiency — since regular wear and tear may affect its accuracy when reading the temperature in your home.

4. Constant Temperature Shifts

A faulty thermostat typically has a hard time maintaining settings. It may continuously change temperature settings without warning.

Try lowering the thermostat settings for a test trial and see what happens. If the same problem occurs, call a technician to check the thermostat.


How to Troubleshoot Your Thermostat

For most of these steps, we’re going to assume you have a digital thermostat. However, we’ll also cover tips for the older thermostats below if you haven’t made the switch from analog to digital.

Here are steps you can take yourself to try and fix a broken thermostat before calling the pros at Ambient Edge:

Check the Power Circuit

Are your HVAC system components receiving power? Check your circuit breakers and make sure they have not been tripped. If so, switch them back on and see if your thermostat functions properly again.

Try Replacing the Batteries

Many digital thermostats are powered by AA or AAA batteries. If the thermostat batteries are worn out, they will need to be replaced with a fresh set. This simple step alone resolves many thermostat issues.

Clean Your Thermostat 

Dust, nicotine buildup, and other dirt inside can all affect your thermostat’s performance. Some thermostats open easily so that you can clean the inside. Others will require that you unscrew the faceplate. 

To clean your thermostat, brush the inside gently with a dry, clean paintbrush or similar soft tool. Compressed air also works if you follow all safety instructions on the can. Always cut power to the thermostat’s circuit and remove batteries before doing this.

Troubleshooting an Analog Thermostat

If your older electromechanical thermostat (usually controlled by a dial) is not working, there are extra steps you might take to find out why. When you open it up, you’ll see a metal strip, often in the shape of a circle or coil. Try lightly pushing it in either direction to see if that solves things. 

You may also need to follow additional instructions in your user’s manual, specifically for setting it to the proper amperage setting. This may not sound easy, but it only takes a few minutes.

Air Temperature Changes

The thermostat can also affect the temperature of the air that flows into your vehicle through the vents. If the air temperature goes from hot to cold or cold to hot, this is a sign that your thermostat is not working properly. Oftentimes, these temperature changes are erratic and sudden. It’s important to have your vehicle inspected when you experience these interior temperature changes.

How to change or test a thermostat

Testing (and, if needed, changing) a thermostat is not difficult, but there are several important precautions to take. First, never crack open your car’s cooling system if the engine is hot; that’s a good way of getting scarred for life. Second, as mentioned above, the housing is sealed by a gasket, which we strongly recommend you don’t re-use — it may not even come out in one piece. Order the correct gasket (and check if it needs sealant as well), wait for the engine to cool down, keep rags handy, and this shouldn’t take more than a few minutes of your time.

On most cars, the thermostat is located near the top of the engine. That’s not always the case: if you drive a Subaru, you may need to crawl under the car to access it. After placing a pan underneath where you’ll be working, remove the hose going to the thermostat housing, loosen the bolts that secure the top part of the housing, take it off (and use a rag to wipe off any coolant that drips), and you’ll see the thermostat. Remove it and carefully scrape off any gasket residue left. If you know that the thermostat is bad, pop the new one back in (make sure it’s in the right direction), fit the new gasket, tighten the bolts, re-install the hose, and you’re good to go. If you’re not sure, head to the kitchen.

If you know how to make pasta, you know how to test a thermostat — just don’t add salt. The main difference is that, when you’re cooking fettuccine you add the pasta after the water starts to boil. When you’re testing a thermostat, you’ll want to drop it in a pan of cold water and then bring it to a boil. We’re assuming that your thermostat is cold and therefore in the closed position; if it’s cold and it’s open you’ve found your problem. Place it in a pan full of water, wait until it boils, and use a ladle to fish it out. It should be in the open position; if it’s not, it’s stuck closed. Wash the pan thoroughly after testing a thermostat, otherwise your next arrabbiata sauce is going to taste a little funky.