I recently realized my temp gauge always stays below where the temp is supposed to be even when I have been driving for a while. While I'm driving the heater can get hot but the temp goes even lower. My AC blows cold. I got a P0128 code stating that the temp is below the thermostats specified temperature. When I was pushing it the other night on the freeway the gauge was going way over temp. Almost all the way hot. When it said it was super hot I tried turning on the heater but it was not blowing any hot air. It wasn't even warm. When I opened the hood and took off the radiator cap it didn't shoot steam out. It barely made any noise. There was also coolant around the coolant reservoir, like it leaked out of the hole that allows it to breath. There was still coolant in the reservoir, I didn't lose a lot. Do i just need to replace the thermostat and the radiator cap? or should I try taking a look at the radiator?

 

Start by pressure testing the system to see if it holds pressure, if not then find the leak and fix it. Sounds like you have a bad thermostat and something leaking not allowing it to build up coolant pressure.