What goes bad in a thermostat?

Author: Valentine Hoeger  |  Last update: Wednesday, December 24, 2025

Wiring can shift and come loose over time. When the wiring between the thermostat and the air conditioner, furnace, or heat pump comes loose, it will break the connection and leave you cold. Wiring defects can also cause the system to misread temperature settings.

What makes a thermostat go bad?

  • Age and Wear: Over time, thermostats can wear out due to repeated heating and cooling cycles, leading to sticking.
  • Debris and Corrosion: Sediment or rust can accumulate in the coolant system, causing the thermostat to stick in the closed or open position.

What would cause a thermostat to burn out?

This can happen if there is a power surge or if the thermostat is connected to a faulty electrical circuit.

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.

What would cause a thermostat to stop working?

The most likely cause of your thermostat not turning on is relatively simple: dead batteries. The solution is equally simple: try replacing the batteries to see if your thermostat powers back on. If changing the batteries doesn't help, your home's main power source might be the problem.

SYMPTOMS OF A BAD THERMOSTAT

How can I test if my thermostat is bad?

When you set the thermostat above or below the current room temperature, you should hear a click, and the AC or furnace should kick on almost immediately. If you make changes at the thermostat and nothing happens, that's the first sign you have a bad thermostat.

Why is my thermostat not working but the fan is running?

The thermostat is blank but the fan works, what does that mean? If the display is not working but the fan is on and your thermostat is in the “auto” position, this is because the display controls the heating and the AC but in the “auto” position, the fan can automatically come on and off.

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 thermostat in a car?

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.

Can a bad thermostat affect AC?

Signs of a Faulty Thermostat

AC Refuses to Start or Stops Suddenly: A thermostat that's not working correctly might fail to initiate the cooling cycle or might randomly shut off the AC. This erratic behavior can be due to electrical issues or miscommunication with the AC unit.

Can the thermostat be bad but not overheating?

A high reading on the temperature gauge could mean any number of problems from a coolant leak, a water pump malfunction, or broken thermostat gauge. It could be displaying an inaccurate temperature. In this case, your car may not be overheating, but your temperature gauge won't be able to warn you.

Can bad batteries affect the thermostat?

Yes, a dying battery in your thermostat can have an impact on your HVAC system. If your thermostat's battery is low, it can cause your HVAC unit to function erratically or potentially not at all. Your thermostat plays a huge role in how your HVAC unit functions as it regulates the temperature of your system.

What happens when thermostat is stuck?

Overheating is the most common symptom of a failing thermostat. Due to corrosion or aging, your car's thermostat can get stuck in a closed position. If this happens, the thermostat will not let the coolant reach the radiator, and, as mentioned above, the engine will overheat, causing severe damage.

Can a bad thermostat cause a blown head gasket?

Blown head gaskets can happen when an engine severely overheats due to a faulty thermostat, clogged radiator, leaking water pump, radiator, radiator hose or heater hoses or when the coolant in the cooling system is not changed per the O.E.M.'s scheduled maintenance.

What causes car thermostats to fail?

Corrosion: Over time, the thermostat can become corroded due to the presence of minerals in the coolant. This corrosion can hinder the thermostat's movement and affect its ability to regulate coolant flow.

Will a bad thermostat cause no AC in a car?

The thermostat is responsible for regulating the temperature of the AC system. If the thermostat is not working properly, it won't be able to regulate the temperature properly, which will cause warm air to be blown into your car.

How will I know if my thermostat is bad in my car?

Temperature Fluctuations: One of the most common signs of a bad thermostat is erratic temperature readings on your dashboard gauge. If you notice that the temperature gauge moves from normal to hot quickly or fluctuates unexpectedly, it's a clear indication that your thermostat may be malfunctioning.

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.

How can I tell if I need to replace my 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.

Will a bad thermostat throw 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.

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.

How long does it take to fix a thermostat in a car?

Car thermostat replacement time is 30 to 60 minutes, depending on the make of the car. It will cost you around $190 to $350, including labor. Thermostat repairs usually take more time, requiring extra time to diagnose and analyze the vehicle's thermostat. It can take up to 2 hours.

Will fan kick on if thermostat is bad?

A bad thermostat can also cause the fan to run continuously or not turn off when it should. To ensure your thermostat is functioning correctly, it is recommended to consult with HVAC Alliance Expert's specialists.

How much is a new thermostat?

Replacing a Thermostat. You'll pay between $15 and $300 for the unit in addition to labor ($65 to $100 per hour) to replace an old thermostat with a new one.

How do you know if your thermostat is not working?

One of the most obvious signs you need to replace your thermostat is when it doesn't do anything. If your thermostat display doesn't light up or respond when you press buttons or try to change the settings, you may have to replace it. Before assuming you have a broken thermostat, make sure it has fresh batteries.

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