What does a thermostat do when it goes bad?

Author: Sophia Koch  |  Last update: Wednesday, October 15, 2025

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.

How can you tell if your thermostat is bad?

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.

Will AC run if the thermostat is bad?

So, can a bad thermostat cause your AC not to cool? 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.

What happens to a car if the thermostat is not working?

A malfunctioning thermostat can impact a car's heating system, leading to inadequate heat or overheating. It regulates the engine's temperature, affecting the warmth provided by the car's heater. A faulty thermostat should be replaced to maintain a comfortable and safe driving environment.

How do you tell if you need a new thermostat?

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

SYMPTOMS OF A BAD THERMOSTAT

What are the symptoms of a thermostat not opening?

High temperature readings

The main symptoms of a failing thermostat are related to engine overheating. It can be easily noticed because the temperature gauge in the vehicle will probably read unusually high. This can be caused by the valves being stuck in a closed position.

Does a new thermostat make a difference?

If you have an older HVAC system, you can optimize and extend its life by getting a thermostat upgrade. This will reduce further wear and tear, thereby eliminating the need for constant repair and other maintenance costs.

Can I drive a car with a bad thermostat?

If you drive with a bad thermostat, your car could overheat, causing severe and expensive engine damage. And while it's technically possible to drive without a thermostat, we don't recommend it. Your car will not reach an acceptable operating temperature, resulting in higher fuel consumption and excessive engine wear.

How much does it cost to replace a car thermostat?

The average cost for car thermostat replacement is $661 to $747. Enter your vehicle's information to see how much thermostat replacement costs in your local area.

How to tell if your thermostat or water pump is bad?

Here are signs you've got a bad pump or thermostat.
  1. Corroded Cooling System Parts. Both of these cooling system parts can get corroded if they start to leak slowly. ...
  2. Leaking Engine Coolant. ...
  3. Erratic Temperature Changes. ...
  4. Poor Coolant Circulation. ...
  5. Overheating Constantly.

Can a thermostat go bad without overheating?

One of the lesser-known issues is the polar opposite of overheating. Overcooling happens when the engine does not reach the right temperature for its normal function. In this scenario, the thermostat is stuck open and overflows the radiator with coolant, drastically reducing the engine life cycle.

How to reset a thermostat?

Most modern thermostats will have a clearly marked reset button inside the panel. It's usually small and recessed, so you might need a pointed object like a paperclip to press it. For those wondering specifically how to reset air conditioner thermostat models, the process is generally the same.

Will a bad thermostat run a code?

While there is a small chance that there is an issue with the running temperature of your engine, by far the most common issue with trouble code P0128 is a faulty thermostat. The exact location of the thermostat and issues related to a damaged thermostat may vary depending on your vehicle.

Will AC work if the thermostat is bad?

The heating, ventilation, and air conditioning (HVAC) system in your home is controlled by your thermostat. If your thermostat isn't working, your furnace, whole-house fan, and AC unit are probably not functioning correctly, either.

How do I test my thermostat?

Methods for Testing Your Thermostat
  1. Just Turn it On and Let it Work. One way to test your thermostat is to simply turn it on and see if it cools the room down. ...
  2. Get an Indoor Thermometer. Another way to test your thermostat is to use a thermometer. ...
  3. Use a Multimeter. ...
  4. What to Do if You Notice an Issue.

Is it hard to replace a thermostat?

Your thermostat instruction manual can tell you exactly how to change a thermostat. Installing the thermostat yourself is an easy task that will typically take about an hour from start to finish.

How do I know if my car thermostat needs to be replaced?

5 Signs Your Thermostat Is Broken
  1. You Notice Erratic Temperature Changes. ...
  2. You Find Incorrect Readings on the Temperature Gauge. ...
  3. Your Vehicle Makes Concerning Noises. ...
  4. You Find Coolant Leaks. ...
  5. You Notice Problems with the Heating System.

How many hours does it take to replace a thermostat?

Installation typically takes about 1 hour.

What happens when your 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.

What is the lifespan of a car thermostat?

However, most car experts recommend car thermostat replacement after 10 years. However, this is an optimistic number. Thermostats are more likely to stop working much earlier than a decade. Some factors that induce an early thermostat failure include harsh weather, rough driving, bad engine or thermostat parts.

What happens when your thermostat is stuck?

When a thermostat gets stuck open, it'll allow coolant to circulate continuously even when it's not needed. Unfortunately, that will overcool the engine and cause it to operate below its optimal temperature range. As a result, the engine will lose fuel efficiency, and its components will experience excess wear.

What damages car thermostat?

Corrosion occurs, among other things if a wrong coolant is used. The corrosion attacks the thermostats aluminum parts that almost "etch away" (see figures 1 and 2). But also mixing of different coolants can cause corrosion. Therefore avoid mixing different types of coolant together.

How can I tell if I need a new thermostat?

7 Signs You Need to Replace Your Thermostat
  1. Your HVAC Keeps Turning On or Off. ...
  2. Faulty Thermostat Readings. ...
  3. Suspiciously High Energy Bills. ...
  4. Constant Temperature Shifts. ...
  5. Thermostat Is Too Old. ...
  6. Thermostat Fails to Respond to Changed Settings. ...
  7. Your HVAC System Short Cycles.

How often should a car thermostat be replaced?

How Often Should You Get Car Thermostat Replacement? Car thermostats eventually fail because of normal tear and wear. However, they still function effectively for up to 10 years before needing a replacement.

How long does it take for a new thermostat to adjust?

Your thermostat should adjust to new settings as soon as you input them into the system. Usually, once you make the change to the settings, you'll hear the HVAC system respond right away.

Previous article
How do you fix lack of ventilation?
Next article
Is no water an emergency?