Why does my low coolant light keep coming on?

Why is my coolant light coming on in my 98 audi A4?

• I was driving my 98 audi a4 today for approx. 15 minutes when i noticed my temp gauge was very high and my red coolant light came on. I immediately exited the freeway and parked. I looked under the hood and there was no visible coolant leaks (none on the ground either). I had a normal level of oil and the coloration was also normal. The level of coolant was also where it should be, between min and max lines. I opened the cap and the coolant looked very cloudy and grey. When i restarted the car the radiator fan was running normally and there was no visible emissions from the exhaust. I drove it about 5 minutes back to my house and watched the temp gauge slowly creep up again slightly past the 3/4 mark. I've read some stuff online about a sensor that can go bad, but im not sure that is the case here. The car seemed to be running normally and there was burning or other smells. any ideas?

• Answer:

  it sounds like the thermostat is shot and you need to get a new one because it controls the heat of the antifreeze and how high the Temperature will reach same thing if your car isn't running hot enough