To test a car thermostat, perform a simple boiling water test.
A bad car thermostat usually gets stuck open or closed. Common symptoms include a temperature gauge that spikes or drops erratically, an overheating engine, weak cabin heat, poor fuel economy, or a lingering check engine light.
To test a car thermostat without removing it, perform the Radiator Hose Test. Start the cold engine and feel the upper radiator hose. It should stay completely cold. Once the engine reaches operating temperature, the hose should suddenly get hot, indicating the thermostat has opened and coolant is flowing.
To verify if your car's coolant is circulating, perform a temperature check by comparing the heat of your upper and lower radiator hoses, or use the heater test. These checks require no special tools and only basic observation.
A Temperature Gauge That Acts Unusual
A thermostat issue often shows up on the temperature gauge first. If the gauge rises above normal, especially in traffic or while climbing hills, a stuck thermostat is more likely. If the gauge stays low even after several miles of driving, the thermostat may be stuck open.
To unstick a car thermostat, the only safe, permanent fix is replacement. Because thermostats contain sealed wax that fatigues over time, they cannot be reliably fixed. However, you can perform a temporary "limp mode" repair to get the car to a mechanic or home.
When the thermostat stays closed, the engine will begin to overheat, potentially leading to extensive engine and cooling system damage. Because of this, we don't recommend driving with your thermostat stuck closed.
A bad thermostat usually causes erratic temperature changes (jumping from cold to hot), a slow-to-warm engine, or weak cabin heating. A bad water pump typically shows mechanical symptoms like coolant leaks from the "weep hole", a whining/grinding engine noise, or a wobbling pump pulley.
To remove an air pocket (or "burp") from an automotive cooling system, park on an incline with the engine cold, and attach a spill-free funnel to the radiator. Turn the cabin heater to maximum heat, start the engine, and let it idle. Massage the radiator hoses to push out trapped air, and top off the fluid.
Trapped air in a radiator—whether in a car or a home heating system—creates an airlock that prevents fluid from properly circulating. The most common indicators are uneven heating (cold spots), unusual gurgling or rushing sounds, and a lack of proper warmth.
Methods for Testing Your Thermostat
How can you tell if a thermostat is bad?
The low voltage at a thermostat isn't hazardous, so you could even use a paper clip.
A bad car thermostat usually fails by getting stuck open or closed. Key warning signs include a temperature gauge that spikes into the red (stuck closed) or stays near cold, poor cabin heating, fluctuating temperatures, or a "Check Engine" light.
To test a thermostat without removing it, the method depends on whether you are checking a vehicle or a home HVAC unit.
When a car's thermostat goes bad, the engine's temperature regulation fails. The thermostat typically gets stuck either closed (trapping extreme heat) or open (preventing the engine from warming up), leading to issues like dashboard temperature spikes, poor cabin heating, or an illuminated Check Engine Light.
Bubbling indicates rising air pressure in the cooling system, which is a sign that the flow of liquid is blocked by a pocket of air. One of the most common causes is a blown head gasket, in which the air pressure inside the cylinder heads is transferred to the cooling system.
Not bleeding the air (or "burping") out of a vehicle’s cooling system leaves trapped air pockets behind. This prevents proper coolant circulation, frequently resulting in dangerous engine overheating, a lack of cabin heat, and erratic temperature gauge readings.
No, bubbling coolant does not always mean a blown head gasket. While it is a common symptom of a failed gasket, the bubbles could also simply be trapped air, boiling coolant caused by an unrelated overheating issue, or a faulty radiator cap.
When a water pump begins to fail, your car will typically display a few distinct warning signs. The most common symptoms include an overheating engine, a visible coolant leak (often bright green, pink, or orange), and unusual noises like whining or grinding from the front of the engine.
To tell if a water pump is circulating, verify flow by feeling for heat inside your cabin, checking the upper radiator hose for pressure, or looking for coolant flowing into the expansion tank. If you are checking an automotive system, do not open the radiator cap while the engine is hot.
Testing a coolant thermostat is straightforward. The most definitive method is the Boiling Water Test. Always work on a cold engine, wear protective gloves, and catch any leaking coolant before starting.
Signs the Thermostat Is Stuck Open
Driving with low coolant for 20 minutes is extremely risky. While it might be possible if the coolant is only slightly low and the drive is entirely on the highway, it often leads to severe, permanent engine damage (like a warped cylinder head or blown head gasket).
The furthest you can drive an overheating car is about a ¼ mile before you risk irreversible engine damage. While some vehicles could still make it as far as 20 miles, we would never recommend driving very far with an overheating engine.