I may just be crazy but something about the thermostat being the problem sounds fishy to me, I will be honest, im not a professional mechanic and I dont know alot about the newer computerized cars, but If I remember right if a thermostat is stuck open then that means the water in the engine is just flowing freely thru the system without anything stopping it to let it get up to a certain temperature. one would think this would cause the water not to heat up, but it still gets warm, try your heater, if the water in the engine wasnt warm then it would not be hot. this being said if the water is warm then the temp guage would be reading something at least. not 0 degrees and throwing a code up. the only thing that would make me think the thermostat was the problem would be if there was some sensor on the thermostat itself telling the computer it is stuck open.
UPDATE: well, apparently i was crazy, it was the thermostat in my g6 as well.
dont exactly understand it but it fixed the problem as well as when i pulled the old thermostat the seal around it was pulled away at one part , would have let all kinds of water pass thru it
UPDATE: well, apparently i was crazy, it was the thermostat in my g6 as well.
dont exactly understand it but it fixed the problem as well as when i pulled the old thermostat the seal around it was pulled away at one part , would have let all kinds of water pass thru it 
