Can a bad thermostat cause my AC to not cool?

Author: Loyal Rice  |  Last update: Wednesday, May 14, 2025

One of the most common reasons why an air conditioner might be running but not cooling a house is due to thermostat issues. The thermostat is responsible for controlling the temperature of your home, and if it's not functioning properly, it can cause your AC unit to work inefficiently.

What are the symptoms of a bad AC thermostat?

A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.

Why is my air conditioner running but not cooling the house?

There are a number of reasons why your air conditioner might be running but not cooling. It could be an issue with the thermostat, the compressor, or the evaporator coils. Or, it could simply be clogged with dust and debris, which is preventing it from working properly.

How do I know if I need a new thermostat for my AC?

One of the most obvious signs that your home's thermostat needs replacing is that it has faulty temperature readings. You'll notice that the indoor temperature reading is nowhere near what it actually feels like.

Does a thermostat have anything to do with AC?

A thermostat is responsible for turning on and off the air conditioner's compressor and fan. The thermostat senses indoor temperatures and uses this data to determine when the rest of the HVAC system needs to run and when to stop; i.e. it's a switch connected to a temperature sensor.

Can a bad thermostat cause AC not to cool?

Will AC stop working if thermostat is bad?

Absolutely. While it may seem like a small part of your HVAC system, the thermostat plays a crucial role in ensuring your home stays cool and comfortable. If you're experiencing issues with your AC not cooling properly, checking the thermostat should be one of your first steps.

Will the AC work without a thermostat?

Your thermostat reads the temperature of the room, then sends an on or off signal to the air conditioner. The AC itself isn't detecting the temperature in your home, so it has to rely on the thermostat.

Will a new thermostat fix my AC?

Replacing your thermostat when you install a new AC system is a great idea. The connection between the thermostat and AC is important to the function of the system. Problems with the thermostat can cause the AC to run more often than it should or not as often as it should.

What are the signs that indicate that a thermostat needs to be replaced?

Signs You Need a New Thermostat
  • Heating or cooling system will not turn off or on.
  • The temperature reading is incorrect.
  • Recent spike in your energy bill.
  • Frequent temperature fluctuations.
  • Thermostat is 10 years old or older.

How do I check my AC thermostat?

How to Test:
  1. Replace the batteries.
  2. Set the thermostat to cool mode and lower the temperature by 5-10°F.
  3. Listen for a click – if there's no response, the thermostat may be faulty.
  4. Try bypassing it by connecting the R (power) and Y (cooling) wires on the control board. If the AC starts, the thermostat is likely bad.

Why is my AC suddenly not blowing cold air?

First, check your filters and coils for dirt and debris that may be impeding airflow. Once you've cleaned the filters and coils, run the air conditioner with just the fan so that the ice accumulation can melt off. If the AC still doesn't blow cold air, it could be that refrigerant levels are too low.

Should you turn off AC if it's not cooling?

When you notice that your air conditioner isn't cooling, your first action should be to turn it off. This might seem counterintuitive—after all, you want your home to be cool. However, continuing to run an AC that isn't effectively cooling can cause further damage to the system.

What is the first thing to check when the AC stops working?

Check these issues when the unit won't kick on: Check the thermostat and adjust the settings if it isn't set to Cool and an appropriate temperature setting to call for cooling. Examine the house circuit breakers for the AC unit and reset any tripped breakers. Check the air filter and replace the filter if it's dirty.

Will resetting my thermostat fix my AC?

Any time you are having an issue with heating and cooling units, such as after a sudden power failure or after your children have had 'control' of the system while you've been away, resetting the thermostat should be the first thing you try when attempting to rectify HVAC system issues.

Can I replace my thermostat myself?

In most cases, installing a new thermostat is a relatively simple do-it-yourself project that can result in improved comfort and convenience and one that can help you save energy.

Will you get a code if your thermostat is bad?

P0128 is an extension of OBD-II code P0 which indicates a problem with your car's coolant thermostat.

How can I test if my thermostat is bad?

7 Ways to Tell Your Thermostat Is Broken
  1. The Thermostat Has No Power. ...
  2. The AC or Furnace Won't Turn On. ...
  3. The Heater or AC Won't Turn Off. ...
  4. The Thermostat Reading Doesn't Match the Room Temperature. ...
  5. The Thermostat Doesn't Respond. ...
  6. The HVAC System Is Short Cycling. ...
  7. The Thermostat Forgets Programmed Settings.

How do I know if I need a new AC thermostat?

You should be able to turn on your heating or cooling system from the thermostat, or change operation back and forth from heating to cooling. If you make changes at the thermostat and get no response whatsoever from your furnace, air conditioner, or heat pump, it could mean the thermostat is bad.

How much does it cost to replace a thermostat?

The average cost of a new thermostat, including installation, ranges in price from $50 to $500 or more based on several factors including the type of thermostat, labor costs, the complexity of the installation, and more.

Why is my AC not blowing cold after replacing thermostat?

Check if the wires are properly connected and look for any signs of damage or wear. If you have a battery-powered thermostat, make sure that you've replaced the batteries recently. If your thermostat isn't receiving power, then it may not be able to communicate with your heating and cooling systems.

What happens when AC thermostat goes bad?

A malfunctioning thermostat will have trouble keeping your home's temperature at the right setting. If there's something wrong with the motherboard, it may change its own settings at random. You can test the thermostat by lowering the settings to see if the problem persists.

Can a bad thermostat cause AC not to cool in house?

When your thermostat is working correctly, you can keep it as cool or warm as you need. A bad thermostat can cause your AC to not cool or heat the room to the desired temperature. Air conditioning concerns are more apparent: Your thermostat is one of the best indicators of your air conditioning system's health.

Should I run my AC if it's not cooling?

Yes, you should turn off your AC if it's not cooling. Continuing to run a malfunctioning AC can strain the compressor and other components, leading to more severe and costly repairs.

Do you have to turn off AC to change thermostat?

Turn Off Power to the HVAC System

The first step in how to install a thermostat is making sure the power is turned off to your existing thermostat. Turn off the power for your heating and air conditioning system at the main service panel. Its circuit breaker should be clearly labeled.

Previous article
What is the least expensive way to roof a house?
Next article
Will lagging stop pipes freezing?