Signs and Symptoms of a Failing Car Thermostat

A car's thermostat is a simple and straightforward device. But if it fails, it could lead to serious problems. Learn more about diagnosing the signs of a failing car thermostat.

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When a Car Thermostat Begins to Fail

Car thermostat problems happen out of the blue. Generally you’ll notice your vehicle begins to overheat soon after starting the engine, or the temperature gauge on your dashboard registers below or way above its normal temperature point.

Car Thermostat Overview

A car’s thermostat regulates the flow of coolant through the engine and is an essential part in the operation of your vehicle. In most cases, an overheating or no-heat condition in your vehicle is due to a faulty thermostat. And since the T-stat price is only about $8, it makes more sense to replace it than to spend hours diagnosing the problem. If that doesn’t fix it, at least you’re only out about two hours learning how to replace a thermostat.

Symptoms of a Failing Car Thermostat

Here are the signs your car thermostat is failing:

  • The temperature gauge reads high and the engine overheats.
  • The temperature changes erratically.
  • The vehicle’s coolant leaks around the thermostat or under the vehicle.

Engine Overheating

Here’s how thermostats work. The manufacturers inject a mixture of ground-up brass and wax into a copper cup called a pellet. Then they slip a highly-polished metal rod through a rubber “O-ring” gasket and into the wax. The pellet is sealed with a crimp ring.

Next, a metal “skirt” is welded around the pellet. When the engine is cold, a spring forces the skirt up against a seat (just like a closed faucet), stopping the flow of coolant. As the engine heats up, the wax melts and expands.

Expansion pressure builds to the point where the wax tries to “spit out” the metal rod. But the rod can’t go anywhere. It’s attached to a “bridge” on the other side of the thermostat.

Eventually the pellet itself moves, overcoming the spring’s pressure. So the thermostat opens and allows coolant to flow. The entire system works well until the metal rod corrodes. That corroded rod damages the rubber seal and the wax leaks out. Once that happens, the thermostat stops opening, coolant stops flowing and your engine overheats. The result can be catastrophic engine failure costing several thousand dollars.

Here are car problems you can diagnose and fix yourself.

Replacing the Thermostat

Fail-Safe Thermostats

If your thermostat fails, don’t just replace it and top off the coolant reservoir. It’s sending you a message of a more serious problem — the corrosion inhibitors in your coolant also failed. So flush the system and add fresh coolant any time you replace the thermostat.

And don’t try to save a few dollars by buying a cheap thermostat (about $5). Premium thermostats (about $15 – this one is on sale!) are built to resist corrosion. The difference in price is small compared with the potential engine damage. Some manufacturers even offer “fail-safe” models that fail in the fully open position to prevent engine overheating.

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