My 2007 has 160k miles and at just over 100k I had toyota change all the fluids. That was about 3 or 4 months ago, its a taxi. I started noticing the heat was only hot above 83, and some times not even hot at high. I thought I was out of fluid so I added to the overflow not wanting to open the cap on a hot motor. It didnt take much, but made the heat come back on above 80 degrees so I did nothing else. Last night I had no heat again after driving about 200 miles. The heat came back after turning it off for a few minutes, but I stopped at a gas station anyway when I got off the freeway. The overflow was full. It was also cold. So were the radiator hoses, but not what looked like power steering hoses next to it. Were those the inverter lines? I check that belt pullie was spinning when the engine was running, it was. It a new belt put on by dealer. The car doesnt show any signs of overheating, and is driven 300 or more miles a day in town and freeway. The motor was of course hot, but not overly so. None of the connected components were hot like I would expect if was overheating. This is something I really need to fix myself due to money shortages. Sounds like a thermostat or waterpump in a standard car, but what about in a Prius? Thanks David edit: I should add that when the radiator cap is off, the heat on, and the engine running, there is no movement in the antifreeze, which is full to the top of the neck but has no pressure.