The thermostat is a crucial part of any combustion engine vehicle’s cooling system. It opens and closes to regulate coolant flow and manage the engine’s temperature. But what happens if that thermostat gets stuck open?

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. The car’s indicators and fault codes will also be triggered.

A stuck thermostat is terrible for your car but read through this guide to discover what happens when it will not close. You’ll also learn why that happens and what you can do to fix it.

What Happens If Your Car’s Thermostat Is Stuck Open?

Your car’s thermostat can get stuck in either the open or closed position. Each one will cause different symptoms to occur.

Here’s what will happen if your car’s thermostat gets stuck in the open position:

1. Engine Overcooling

The first thing that will happen when your car’s thermostat is stuck open is the engine will experience an overcooling condition. That’s because the coolant circulates throughout the engine’s cooling system even when it’s not needed.

Remember: all engines have an optimal operating temperature range. Exceeding that range causes the engine to overheat, leading to internal damage. However, an engine cannot operate efficiently if it is too cold, either.

So, since the thermostat won’t close even when it should, your engine will never become hot enough to function optimally. Instead, any heat the engine produces will quickly get absorbed by the coolant and taken to the radiator to release into the surrounding air.

Meanwhile, the blue low-temperature indicator on your instrument panel will turn on to confirm that your engine is still cold. You’ll know there’s a problem when the indicator refuses to shut off even after you’ve been driving for an extended period.

2. Indicators And Fault Codes

A stuck thermostat will also trigger several indicators, warnings, and fault codes. As you read above, you’ll first notice the blue low-temperature indicator that constantly stays on.

However, your car might also turn on other indicators like the Check Engine light. That happens because the vehicle can sense the engine is not operating correctly, which is a side effect of the stuck thermostat.

Meanwhile, plugging into the onboard diagnostics system will likely show you fault codes related to the engine, the cooling system, or the thermostat itself. These fault codes will not go away until you’ve resolved the issue and cleared the codes.

3. Lower Fuel Efficiency

When the engine runs below optimal temperature, its ability to burn fuel efficiently is also undermined. As such, the car’s overall fuel efficiency will drop.

In other words, you’ll find that your car burns more fuel than usual in the long run.

Naturally, this is a side effect that’s not immediately noticeable. But if you let the stuck thermostat remain that way for too long, you’ll also be paying much more than usual for fuel.

4. Excess Wear On Components

Fuel efficiency isn’t the only thing that suffers when your thermostat is stuck open.

Remember: the engine is designed to function within an optimal temperature range. That means temperatures that are too high or too low will cause adverse effects on the engine.

The same is also true for the engine’s components and its fluids. Lubricants and engine oils can only deliver their intended benefits if the engine is hot enough.

Ask any student from a top automotive college, and they’ll tell you that when an engine is being overcooled, its internal components will suffer excess wear. Of course, they won’t break overnight, but the added stress they experience will shorten their overall lifespan.

Suppose you left it that way for several months or longer. In that case, you can expect to deal with much more expensive repairs and parts replacements later.

5. Heater Doesn’t Work

The final thing that’ll happen if your car’s thermostat is stuck open is that the heater won’t work. But, of course, this likely won’t be a problem for most people driving in the hot Malaysian climate.

But let’s suppose you’re driving around Genting Highlands, where it’s approximately 14 degrees Celsius, with a car that has a built-in heater. If your thermostat is stuck open, you won’t get any hot air in the cabin when you turn the heater on.

That’s because the heater gets heat from the engine through the cooling system. Usually, the heated coolant will pass through the heater core and blow that heat into the cabin.

That won’t happen if the thermostat is stuck open, as the engine will never get hot enough to produce and share its heat with that heater core.

Why Does A Car’s Thermostat Get Stuck?

Now that you know what happens when a thermostat gets stuck open, let’s look at why that happens.

Your car’s thermostat gets stuck open because:

Worn-out thermostat: Firstly, your thermostat might get stuck simply because it is too old and worn out. You’ll likely find that the thermostat is rusted on the inside if you inspect it.
Weak spring: Thermostats rely on a spring to open and close correctly. If the spring is of lower quality, it’ll weaken prematurely and fail to close the thermostat. As a result, the thermostat remains stuck in the open position.
Contaminated fluids: The thermostat essentially works as a gateway for coolant that flows through the cooling system. Excessive contamination of that fluid can cause deposits around the thermostat, causing it to get stuck.

Thermostats generally last around 10 years before needing a replacement. However, inspect your thermostat periodically to ensure it’s still in excellent working condition.

How Can You Fix A Stuck Thermostat?

You can try to clean or knock on a thermostat to get it unstuck. However, that’s only a short-term fix you should not rely on.

Remember: a stuck thermostat is terrible news for your engine, regardless of whether it’s stuck open or closed.

So, the best thing you can do is replace your stuck thermostat with a new one. Not only will it work better, but you can trust that it won’t fail anytime soon.

Final Thoughts

Your engine’s cooling system is critical for stopping your engine from overheating. However, it can also cause problems with a stuck-open thermostat that cools the engine down excessively.

A stuck thermostat should be inspected and replaced with a brand-new one to restore your peace of mind.

Suppose you have a passion for engines and other automotive systems. In that case, you should check out TOC Automotive College, the best automotive college in Malaysia. Choosing to study in an automotive college can help you build your passion for automobiles and pursue a career in the industry!

What Happens If Your Car's Thermostat Is Stuck Open