When and How to Test your Car's Thermostat

Most gasoline and diesel engines include a water-based cooling system to remove excess heat caused by combustion. A coolant circulating through the walls of the engine picks up heat and carries it into the fins of a radiator where moving air helps dissipate the heat.

Cooling systems are simple compared to engines. They include the water jacket, a network of passages though the engine block; the radiator and cap; a fan; a water pump; hoses and the thermostat. Though it's a mechanically simple device, the thermostat serves an important function, regulating the temperature of coolant and engine.

How a Thermostat Works

A typical thermostat. 

A thermostat sits across the opening where coolant enters the block. When the engine is cold, the thermostat closes off the opening to keep water isolated in the block. As the engine begins to generates heat. The thermostat remains closed until coolant inside the block reaches the operating temperature at which the engine performs its best. Then the thermostat opens to allow circulation of coolant from the remainder of the system. It opens wider as the coolant temperature increases: the more coolant that passes through the block and the radiator, the more effectively the system removes unwanted heat. When operating temperatures change, the thermostat can adjust to maintain the coolant's temperature within a narrow set range.

How Thermostats Fail

When it reaches the end of its useful life, a thermostat usually sticks in the open position, allowing coolant to circulate through the system continually. When this happens, an engine is unlikely to reach its optimum operating temperature, which both increases wear and erodes fuel economy. In cold weather the heater, which is in most cars part of the cooling system, will function poorly or not work at all. More rarely, the thermostat will get stuck in its closed position. In this case, coolant in the system won’t circulate through the engine block, causing the engine to overheat.

Track Down an Electrical Circuit on a Breaker Panel

Remember the last time you did electrical work around the house? Maybe you installed a ceiling fan, put up a new security light, swapped in a dimmer switch for a toggle, or perhaps you tackled a more ambitious project like adding a new fixture. I can pretty well guarantee the first line in the instruction sheet said words like: 

"Turn off the power to the circuit before starting." 

Oh, sure. That's easy for the engineer who designed your new toy to say: he's probably posted a blueprint on the inside of the door of his breaker box; a tiny map showing the location of every fixture, switch and outlet in the house, everything carefully labeled with the number of its circuit breaker. If you're like everyone else, though, the breaker box is probably labeled with cryptic little notes like "mr bth ot," "dck lts," or "sara's bdrm" - and the only Sara you know is the intern at work.  If you're really unlucky, the breaker isn't labeled at all.