Re: I know I have to replace my thermostat but this
Thermostat open/closed doesn't make the heat work or not. The heater core is on the "inside loop" and gets coolant circulation regardless of the position of the stat. You've got something else going on too - air pockets, temp sensor problems? Jumping from normal to red in 1s isn't what would be expected and might be a false reading. You need to get that code read too as it may point you in a good direction.