How Do I Know If My Truck Thermostat Is Bad?

One of the common questions truck owners ask is how do I know if my truck thermostat is bad? Since a bad thermostat can mimic other vehicle issues, it can be hard to figure out exactly what’s going on. Keep reading to learn about some things you should look for when trying to figure out if your truck thermostat needs attention.

How Do I Know If My Truck Thermostat Is Bad?

A broken thermostat can cause several different symptoms, from simply the engine light coming on to rumbling and overheating. All of these symptoms are a result of your engine failing to regulate it’s operating temperature, which can result in a number of different, but usually pretty obvious tell-tale signs. Below is a list of some of the most common signs that your thermostat is broken.

Lower or Higher Temperature Gauge

One of the biggest signs that something is going on with your thermostat is the engine running hotter or colder than normal. Normally, the temperature gauge should sit right in the middle between hot and cold or more towards the cold side.

When you start noticing that your gauge isn’t sitting where it normally does, you’ll want to make sure you get it looked at as soon as possible to reduce the risk of causing more damage. 

Keep in mind, if your truck only has this one symptom, then it might not be the thermostat. This is especially true if you don’t notice any other issues happening, but you should get your truck checked out regardless.

Erratic Temperature Changes

Another common symptom of a faulty thermostat is if the temperature of the engine changes erratically. For example, one minute you might glance at the gauge and see that it’s right where it’s supposed to be and another time you might notice it’s leaning more towards the cold or hot options. It might not seem like a big deal, but it can cause a lot of issues to happen with your engine.

If your truck is having these erratic temperature changes, you really shouldn’t wait to get it looked at. Letting this happen for a prolonged amount of time can cause more damage and end up costing quite a bit in the long run.

Coolant Leaks

One of the most common signs of an issue in your truck’s thermostat is leaking coolant. Sometimes, you’ll see a puddle under your car and other times you’ll only see the leaking fluid around the hose or thermostat. This can happen for a few different reasons, including the thermostat being stuck in the closed position or staying in the open position.

Getting this looked at as soon as possible is essential because leaving it can cause other coolant hoses to start leaking and cause more damage in the long run. One thing to keep in mind is you might see coolant leaks if you’ve just refilled the reservoir or did a complete coolant replacement.

Decreased Fuel Economy

Before talking about why your gas mileage drops with a faulty thermostat, it’s important to remember that this symptom could be from a different issue. With that said, though, it’s important to get your truck looked at as soon as you notice a change in your gas mileage.

The main reason this happens when you need to replace or repair your thermostat is that it will cause your engine to run richer, which decreases your gas mileage.

It’s important to remember that a small drop in gas mileage is completely normal, but any big jump in numbers could mean that your thermostat is giving out. Try and get your truck into a shop as soon as possible to see what your next steps are.

Shifting Issues

If your truck is automatic, then a big symptom that something might be going on with the thermostat is trouble shifting to higher gears. This happens because the engine doesn’t have enough power to smoothly shift.

Some trucks won’t shift at all, while others will get stuck then shift hard. Both of these situations could cause your engine to blow up, so you’ll want to get into a mechanic as quickly as possible.

Keep in mind, this symptom doesn’t automatically mean that your thermostat is the culprit, but you’ll have a good idea of what’s going on and what your next steps need to be in order to get your truck back in working condition.

Temperature Warning Light Engaged

Most of the time, one of the first indicators that your thermostat could be faulty is the temperature warning light coming on. This little light is located on your dash and it only comes on when your engine is running too cold or too hot. While there could be other reasons for this light to come on, a large number of people find that the issue gets fixed once the thermostat is replaced.

Keep in mind, driving when your temperature light is on could cause irreversible damage to the engine and other important parts of your truck. As soon as you notice the light on, you should stop driving and get into a shop as soon as possible.

Air Temperature Changes

Another common symptom of a faulty thermostat is if you notice the temperature inside of your vehicle changes rapidly. Most people will notice that the temperature either raises or drops a few degrees at first, then the temperature will shoot up or drop very quickly. If you notice these changes and haven’t messed with your settings, then there’s a high chance that something is going on with your thermostat or radiator.

This particular symptom isn’t as serious as some of the others on the list, but you should still get into a mechanic as soon as you can. This will allow you to find out what your next steps are and reduce the chances of causing even more damage.

Rumbling Noises

Once you notice that your temperature is changing, many people will start hearing a rumbling noise as they drive. Since this can happen as a result of countless other issues, you’ll want to get into a mechanic to pinpoint the problem.

These noises can come from many different areas, including the thermostat, engine, or radiator. It’s important to know that the sound might not be a rumbling, but could sound like boiling, knocking, or gurgling.

All of these sounds aren’t usually normal for a truck, so figuring out the issue is essential. If you have the knowledge, you can do this at home but those that aren’t sure what they’re doing should make an appointment with a reputable mechanic.


The final symptom that many dirvers notice is that their truck starts overheating. As stated above, the temperature gauge should sit right in the middle or more towards the cold side. Depending on the situation, it’s normal for your engine to heat up a little bit, but it becomes a problem when the gauge goes all of the way into the red.

When this happens, you’ll want to stop driving immediately because it can cause your engine to blow up, a head gasket to blow, or your car to completely stop running.

After turning the car off, you’ll want to get it towed to your home or a shop. Don’t try to drive to your location because possible damage could end up being quite a bit more expensive than the original issue.

By knowing what to look for, you’ll have an easier time figuring out what the exact problem is. If you are able to narrow it down to your truck thermostat, you can either do the work yourself or take it to a reputable shop. If you still aren’t sure what the issue is, get your mechanic to give it a look over.

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