Hey Scotty, I have an 03 f150 with heat issues. I replaced bad thermostat and flushed out my heater core. But still intermittently have no heat. Am I getting air in the system without leaking any fluid? The level isn’t changing when cold 1/2 inch above cold level line. Any help would be appreciated.

air will make it do that, first have all air bled out of the cooling system. If not that, realize the heating system on that is all computer run and often those systems break down, requiring a pro mechanic with a dealer level scanner to fix them