Thermostat Stuck Open Symptoms


The thermostat is a small component in a car’s cooling system that plays a crucial role. It opens and closes, allowing coolant to flow and preventing the engine from overheating. But what happens when it gets stuck open, and how will you know it’s happening?

You’ll know your car thermostat is stuck open when the engine becomes too cold or takes too long to warm up. Besides that, you’ll find that your heater doesn’t produce any heat, and the vehicle’s fuel efficiency drops over an extended period. Simultaneously, the Check Engine light will turn on, and error codes will appear on the onboard diagnostics.

Overcooling caused by a thermostat stuck open is not something to be taken lightly. So, read through this guide to learn its symptoms and discover what you can do to fix it.

What Is A Thermostat, And How Does It Work?

The combustion engine in your car runs hot enough to overheat and damage itself. That’s why the engine works closely with the vehicle’s cooling system consisting of a radiator that circulates coolant and keeps the engine within safe operating temperatures.

Many people don’t realize that the cooling system only circulates coolant when the engine reaches a high temperature. That way, the system avoids overcooling the engine and causing it to lose efficiency.

The component that determines when coolant flows or stops is the thermostat. You can think of it as a temperature-controlled valve in the cooling system that only opens when the engine becomes hot enough.

Doing so allows coolant to circulate and keep the engine within safe operating conditions. Once the engine is cool enough, the thermostat will shut and stop the coolant from flowing.

For the engine to stay at the correct temperatures, the thermostat must be able to open and close without getting stuck in either position. A thermostat stuck in the closed position will quickly lead to engine overheating. In contrast, one stuck open will cause excessive cooling for the engine.

Read: How To Prevent Air Bubbles From Getting Into Your Car’s Cooling System

What Are Symptoms Of A Thermostat That’s Stuck Open?

It’s much easier to know when a car’s thermostat is stuck in the closed position because the engine will quickly overheat. That’s not a symptom you can overlook, even if you tried.

Unfortunately, it’s not so easy to notice when a thermostat is stuck open, and the engine is overcooling.

Here are 5 symptoms you can look for to know if your thermostat is stuck open:

1. Check Engine Light and Fault Codes

One of the first signs you’ll see that your thermostat is stuck open is your Check Engine light turning on. But, of course, that light can turn on for a long list of very different reasons, so there’s no way to know that it’s being triggered by your thermostat specifically.

However, cars these days feature plenty more sensors than ever before. These sensors allow the vehicle and its computer to monitor everything going on and warn you when the cooling system is malfunctioning, such as with a stuck thermostat.

So when you see that your Check Engine light is on, you can take that as a prompt to check the onboard diagnostics. Using an OBDII scanner will pull up any active error codes, which will tell you whether or not you have a problem with the thermostat and cooling system.

Read: What Happens When Your Car Overheats?

2. Engine Temperature Drops

You can also tell that the thermostat is stuck open when the engine temperature drops, even though the car’s been running for quite some time.

Some cars have engine gauges that can show you their precise temperatures at any given time. But even if your vehicle doesn’t have that, it’ll certainly have the indicator light that tells you the engine is still cold.

That light should never come on while the car is running. So, when it does, that’s an indicator that the thermostat is causing an overcooling condition to occur because it’s stuck open, letting too much coolant flow through the cooling system.

Read: Why Radiator Bubbling With Cap Off?

3. Engine Takes Too Long To Warm Up

All engines take a few minutes to warm up after starting, especially if you’re doing so first thing on a cold morning.

Still, if you’ve driven that car for an extended period, you’ll have a clear sense of how long it takes to warm up.

When the car consistently takes much longer than usual to warm up and turn off the cold engine light, that’s another symptom of a thermostat stuck open.

4. Heater Produces No Heat

Suppose you drive your car in a cold climate or during the winter months. You’d likely turn on your car’s heater to stay warm inside the vehicle’s cabin.

The heat supplying that heater comes from the engine through the radiator and the heater core. In other words, the car’s heater takes advantage of the engine’s heat to keep you warm inside during the winter.

A thermostat stuck open will prevent that heat from ever reaching inside your car, so you’ll still be cold despite turning the heater on.

Read: Why Radiator Keeps Pushing Water Out?

5. Fuel Economy Drop

One thing to always remember is that your car’s engine must be hot to run efficiently. An overheating engine is dangerous, but an overcooling one won’t be able to function correctly.

Part of the problem is that a cold engine causes fuel to condense inside its cylinders. As a result, the fuel and air mixture inside the chamber can’t combust correctly, and the engine fails to produce the power it needs for your vehicle to move.

Can You Drive With A Thermostat Stuck Open?

No, you should avoid driving if you know the thermostat is stuck open. In fact, you should keep the engine off to protect it from excess wear.

Thankfully, the problem of a stuck thermostat can be fixed no matter where the vehicle is parked. Therefore, you don’t have to tow the car to a workshop or need any heavy-duty workshop tools.

Read: Why Car Overheats?

What Do You Do If Your Thermostat Is Stuck Open?

When your thermostat is stuck open, the most practical solution is to replace it with a new one. That’s especially true if you’ve been using that thermostat for years.

A typical thermostat will fail after about a decade. That’s excellent news because it means this isn’t a part you have to troubleshoot, repair, or replace often.

So, once it fails, you’re better off replacing it with a brand-new one. Choose a high-quality one from a reputable brand so that you can install it and not worry about it for many years to come.

When it comes time to replace the thermostat, shut the engine off and allow the radiator to cool for at least 30-60 minutes.

Remember: the cooling system absorbs the engine’s heat, so it’ll be dangerous to work on while it’s still hot.

Final Thoughts

A thermostat stuck in the open position is not as dangerous as one stuck shut. However, a closed thermostat will lead to overheating, quickly damaging the engine from the inside.

However, a thermostat stuck open will lead to overcooling. Still, an engine that’s too cold won’t run efficiently, and its parts will experience excessive wear.

So, while the damage is slow, it nevertheless leads to engine damage in the long run.

Thermostats don’t fail often, so when yours does, the best thing you can do is replace it with a brand-new one immediately.

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