What is the opening temperature of the thermostat?

There is no single universal opening temperature for thermostats. In automotive engines, the thermostat typically begins to open in the mid-80s to mid-90s Celsius, with many designs aiming around 90°C. Exact values depend on the engine and manufacturer.

Understanding this threshold matters because it affects how quickly an engine warms up, how steadily it runs, and how efficiently the cooling system manages heat. This article explains what the opening temperature means in practice and what values are common for automotive applications, plus how to find the precise spec for your vehicle.

What the opening temperature means

The opening temperature is the point at which a thermostat’s valve starts to move open as the coolant heats up. It is set by the thermostat’s design—often a wax pellet or a spring mechanism—and is chosen to balance rapid warm-up with stable operating temperature and efficient cooling.

Typical automotive thermostat opening temperatures

Before listing ranges, keep in mind that exact figures vary by engine, climate, and design. The ranges below reflect common specifications in modern gasoline engines:

• Opening temperature commonly around 82–88°C (180–190°F).


• Full opening or maximum flow typically reached around 88–95°C (190–203°F).


• Some engines use higher or lower values depending on cooling system design and performance goals.

For anyone replacing a thermostat or diagnosing cooling issues, consult the vehicle’s service manual or check the stamped specification on the thermostat itself to confirm the exact opening temperature for your engine.

Summary

In short, there is no universal opening temperature. Most automotive thermostats begin to open in the low to mid-80s Celsius and reach full opening in the low to mid-90s Celsius. Always verify the exact spec for your specific vehicle model.

How to tell if a thermostat is opening?

You can tell a thermostat is opening by observing engine temperature, using an infrared thermometer to check hose temperatures, or checking coolant circulation in the radiator. A correctly opening thermostat will allow the engine to reach its operating temperature, after which the upper radiator hose becomes hot, and coolant can be seen circulating in the radiator or its clear overflow tank.
 
This video demonstrates how to check if your car's thermostat is working by checking the radiator hose temperature: 57sYou WrenchYouTube · Mar 29, 2021
How to check the thermostat
Method 1: Check radiator hoses

1. Start your engine and let it run until it reaches normal operating temperature. 
2. Carefully feel the upper and lower radiator hoses. 
3. The upper hose should be hot, as it's carrying hot coolant from the engine to the radiator. 
4. The lower hose should also be warm, as this indicates the thermostat is open and coolant is circulating through the entire system. If it remains cold, the thermostat may be stuck closed. 

This video shows how to use an infrared thermometer to check the temperature of the radiator hoses to see if the thermostat is working: 36sHVAC Mechanic YouTube · Jun 24, 2023
Method 2: Check for coolant circulation 

1. Start the engine and allow it to warm up.
2. Check the radiator or the clear coolant overflow tank.
3. You should be able to see coolant circulating in the radiator or overflow tank once the engine reaches operating temperature, which happens when the thermostat opens. 

What a thermostat stuck open or closed means

• Stuck closed: The engine may overheat because coolant cannot circulate to the radiator. 
• Stuck open: The engine may not warm up to its proper operating temperature, which can affect engine performance, fuel efficiency, and cabin heat. 
• OBD-II scanner: A scanner can also indicate a stuck thermostat by reading engine temperature data, says AutoZone.com. 

You can watch this video to learn how to diagnose a thermostat stuck open: 1mEasyAutoFixYouTube · May 24, 2021

What is the opening temperature of a thermostat?

A car's thermostat opens when the engine coolant reaches the temperature rating stamped on the thermostat, which is typically between 180 and 195 degrees Fahrenheit for most vehicles. The thermostat will begin to open at this temperature and will be fully open at a higher temperature, about 15–20 degrees above the opening rating. 

• Opening temperature: This is the specific temperature, often stamped on the thermostat itself, at which it begins to open to allow coolant to flow to the radiator. 
• Full opening: The thermostat is not fully open at its stamped temperature. It takes a range of about 20∘F20 raised to the composed with power cap F20∘𝐹 to open completely. For example, a thermostat rated at 180∘F180 raised to the composed with power cap F180∘𝐹 will begin to open around 180∘F180 raised to the composed with power cap F180∘𝐹 but won't be fully open until it reaches approximately 200∘F200 raised to the composed with power cap F200∘𝐹. 
• Operating temperature: The engine's normal operating temperature is the temperature it is designed to run at, and the thermostat helps maintain this range by regulating the coolant flow. A thermostat's rating should be based on the vehicle's original equipment manufacturer (OEM) specifications. 

How to check thermostat opening temperature?

Place the thermostat in a pot of water on the stove. Turn on the heat and use a thermometer to monitor the water temperature. As the water heats up, observe the thermostat. It should start to open around the specified temperature (usually marked on the thermostat).

What should my thermostat be set at in October?

So, What's the Ideal Temperature in the Fall? There's no one-size-fits-all answer, but here's a good baseline: Set your thermostat between 68°F and 70°F when you're home and awake. This range keeps most people comfortable in sweaters or light layers, without making the furnace work overtime.