The thermostat limits the flow of coolant through the engine. This reduces the engine's time to warm up and helps maintain the optimum temperature once it’s reached. Through thousands of heat cycles, the plastic in these units weakens. The result is often cracked and leaking housings. Thermostats can also often become stuck in one position, resulting in engine overheating or running too cool. This relatively inexpensive item should be on everyone’s list of items to replace every few years, so as to maintain optimum performance.