Blog Car Care Advice 7 Bad Thermostat Symptoms to Watch For (+How to Replace It)
Car Care Advice

7 Bad Thermostat Symptoms to Watch For (+How to Replace It)

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Your car’s thermostat keeps your car running at peak efficiency on the road.

However, a failing thermostat can overheat or overcool the engine, leading to many other dangerous issues and expensive repairs.

But how can you know if your car thermostat is failing? 
Read on to learn about the bad car thermostat symptoms. We’ll also cover the thermostat replacement steps and associated costs.

This Article Contains:

7 Bad Thermostat Symptoms to Look Out For

Here are the common symptoms of a malfunctioning thermostat:

1. Illuminated Check Engine Light

Your check engine light may be one of the first symptoms of a bad thermostat that you’ll notice — illuminating on the dash with or without an associated code.

You may also see a P0128 error code (Faulty Engine Coolant Temperature). This is normally attributed to a thermostat problem or issues within the engine control module, leading to the engine running too cold. 

2. Erratic Temperature Changes

A sudden cabin temperature change (from hot to cold or cold to hot) is a clear sign of thermostat failure. This happens because a faulty thermostat will affect the temperature of the air flowing through your vehicle vents.

3. Coolant Leaks

When your thermostat stops working, you may notice a higher incidence of leaking coolant. This happens when your engine’s operating temp is too high due to dysregulation, causing an overflow of coolant into the surrounding areas. 

It can result in a “sweet scent” when you operate the vehicle or white smoke from the exhaust. 

Note: You should also check for issues with the head gasket or radiator cap.

4. Bad Temperature Gauge Reports 

Your engine coolant temperature gauge (coolant temp gauge) can help you spot a broken thermostat. If you frequently notice high or low-temperature gauge readings that are out of the norm, it may be time for a vehicle thermostat inspection. 

Catching a faulty thermostat early can save you significant costs since it can impact the engine’s normal operating temperature. An optimal engine temperature plays a key part in its lifespan and performance.

5. Rumbling or Knocking Noises

If your thermostat isn’t functioning properly, your coolant temperature may hit boiling point, resulting in a banging or rumbling sound from your engine bay. This will be noticeable as you drive and can worsen as you accelerate. 

If these knocking noises are paired with a sweet, burning scent or white smoke from your exhaust, it could be due to a thermostat-related coolant leak. 

6. Overheating or Overcooling

A thermostat valve can get stuck in a closed position due to corrosion or aging. A closed thermostat won’t let hot coolant reach the radiator to dissipate heat. This causes the engine’s temperature to rise, and the engine will overheat. Overheating can lead to severe engine damage.

However, if a thermostat valve gets stuck in an open position, excess coolant gets to the radiator — causing overcooling. In this case, you might face heater problems, and the engine won’t reach an optimal temperature, affecting its performance

7. Increased Fuel Consumption

Thermostat failure means the engine won’t run at its appropriate operating temperature — resulting in either engine overheating or overcooling. Both can eventually lead to reduced fuel economy and increased exhaust emissions.

Now that you’ve confirmed you have a failing thermostat, let’s see how to fix it. 

How to Replace a Bad Thermostat

A thermostat replacement isn’t a complicated process. However, you should know where it’s located (typically where the upper radiator hose joins the engine block) and be confident about handling the process.

If you can’t locate the thermostat or don’t want to risk a bad replacement, it’s best to leave it to a professional mechanic

Here’s how a mechanic would do it:

  1. Keep a large enough container or bucket ready to collect escaping coolant.
  2. Remove the clamp that connects the upper radiator hose (coolant hose) to the radiator.
  3. Disconnect the radiator hose and collect the escaping coolant from the radiator in the bucket.
  4. Unscrew the bolts on the thermostat housing and the old thermostat.
  5. Take off the gasket around the old thermostat housing.
  6. Remove the broken thermostat.
  7. Install the new thermostat (spring-side down) with the bolts.
  8. Reconnect the hose and clamp (use a new coolant hose if the current one is old or damaged).
  9. Add the coolant back into the radiator reservoir (if it isn’t old or contaminated).

Next, let’s see how much a replacement costs. 

How Much Does a Thermostat Replacement Cost?

Replacing a failing car thermostat can cost anywhere from $200 to $750, depending on your vehicle model and mechanic’s labor rate

However, if you’re attempting a DIY repair, here are estimates for associated parts and fluid without labor costs:

Still have questions about the replacement?
Read on to find answers.

3 FAQs about Thermostats

Here are answers to some queries you may have about thermostats:

1. What Does a Thermostat Do? 

The thermostat is a part of the HVAC system (Heating, Ventilation, and Air Conditioning). It regulates coolant flow to keep the engine at an ideal operating temperature.

If the engine is cold, a closed thermostat will help it warm up by keeping hot coolant around it. When the engine heats up, the thermostat gradually allows coolant to reach the radiator and dissipate the heat so the engine stays at a normal operating temperature.

However, a failing car thermostat won’t respond as needed, and the engine won’t run at an appropriate operating temp. 

Note: A thermostat problem can lead to a cabin temperature change, but it won’t affect the air conditioner. The car thermostat and air conditioner operate independently despite both being a part of your car’s cooling system. Moreover, air conditioning uses refrigerant, which is different from coolant. 

2. Can I Drive with a Malfunctioning Thermostat? 

No, you shouldn’t. A bad car thermostat can result in cabin and engine temperature changes. While a temperature change may not seem serious, it can lead to costly engine damage and failure.

This can happen if your engine overheats or runs too cold.

3. How Can I Tell if My Car Thermostat Is Working?

If your vehicle is under ten years old, your thermostat is likely working fine. Vehicle thermostats have an average lifespan of a decade. So, those who drive older models should consider investing in an inspection and possible thermostat replacement. 

If you feel comfortable testing your car’s thermostat function at home, here are a few steps to do so:

  1. Start your car and let your car idle for 10 to 20 minutes so that the engine reaches its optimal temperature. 
  2. Open the hood and see if you can see the coolant going through your radiator filler neck. 
  3. If you see a normal coolant flow and don’t notice signs of a bad thermostat (such as knocking sounds or heater problems), your car’s thermostat works as it should.

You could also ask a mechanic to do a quick test, where they’ll place the thermostat in a bucket of boiling water. If the thermostat is fine, it should open when submerged and close when pulled out.

Fix Bad Thermostat Woes with AutoNation Mobile Service

A bad thermostat can lead to irreparable engine damage and expensive repairs if left unaddressed. On the other hand, replacing a malfunctioning thermostat promptly can cost you much less. 

That’s why you should consult a mechanic immediately after noticing any of the above bad car thermostat symptoms. 

Worried about overheating your car while taking it to a mechanic? 
Let the experts at AutoNation Mobile Service come to you!

We’re a mobile auto repair and maintenance company that offers top-quality service with a 12-month, 12,000-mile repair warranty.

Contact us today to have your thermostat or HVAC system issues resolved right from your driveway.