Auto Repair & Diagnostics - How to know if a thermostat is faulty on a car

Hello, my name is Tom Brintzenhofe. I'm a certified master mechanic out of Reading, Pennsylvania. Today, we're going to talk about, how to know if you've got a faulty thermostat in your car. The easiest way to tell, the thermostat will give you two different problems. Either one, your car is going to run too hot and overheat, and two, it will run too cold, and you get no heat. Now, a thermostat sticks closed, you're not going to get any flow going through the motor. It's going to overheat on you. The second thing it's going to do, it's going to stick open, and you're actually getting no heat, because it's going to constantly circle, and not build any heat inside the engine. The way to tell whether or not you have a faulty thermostat, you have an upper and lower radiator hose. After the engine warms up, about 195 degrees, you obviously either, if it's stuck open, don't do this if it's stuck closed, but if it's stuck open, and you're getting no heat, here's a good way to tell if you've got a bad thermostat. Place one hand, with the engine not running, on the top radiator hose. The other hand, if you can reach it, you might have to get up underneath it. Put your hand on the lower radiator hose. They should be about the same temperature. If they're not, obviously, you've got yourself either a low radiator, or stuck thermostat, but they should be about the same temperature, when this thing gets up to about 195 degrees. Just check your two hoses. If the bottom hose is a lot cooler, it's usually stuck. It's real simple. It's not a hard thing to diagnose. Here's a thermostat. It's spring activated. As it heats up, it opens. As it cools down, it will close.