Why did a P1285 code set when the temp gauge never went to over heat?

Question

I have a F150 2003 that is a 5.4 liter, why is the 1285 set never and the temp gauge never indicated over heating?

The issues the came up were P-1285, P0301, P0316,
Trouble shooting I have done thus far is:
checked for external coolant leaks, none found
changed spark plugs
bore scoped each cylinder normal carbon build up on cylinders-none are washed clean
when on highway heat is ok. when stop at stop sign, heat blows cold. temp gauge fluxuates between 1/2 and 3/4
coolant was low-by 3/4 of gallon-dont know why?
Pressure tested system-it held pressure
coolant came out of cap on expansion tank-sprayed left side of truck (good size spray)
heater hoses are hot.
radiator hoses are cold
I think it is a thermostat but dont know where the initial 3/4 of coolant went-why it was low?????? Do you think P1285 set the head gaskets have compressed and a future leak will develop?

Comments

hey out there...anyone ??? can anyone give me their 2 cents please??? thanks!!!!