I have a 2000 with 4 cylinder 2wd ranger with a heat issue that just started. Replaced the thermostat yesterday but go to the store today and no heat. The 2 pipes coming up to the 4 way valve from the engine are hot but then only 1 of the 2 going to the firewall is hot so that would make me think its the heater core but the flow chart doesn't work for my scenario.

1 going to firewall is the hot one and the one is cold so wouldn't mean that its the heater core? I assume the cold one would be the return from the heater core.

But then I go to drive the truck home and the heat works fine. When it doesn't work, it doesn't work meaning its not even slightly warm. Also a year ago I replaced the radiator with new fluid and I have no coolant leaks.