Why would no warm air come out of any vents in a 2000 Nissan Pathfiner with automatic climate control?

There could be numerous reasons, but the easiest ones to check are low coolant in the radiator, not just the overflow tank, or most probably a stuck open thermostat. You also may need to flush the heating core, you should consult the Pathfinder Service manual on how to do this. Basically the idea is to run water through the heating core backwards until it comes out clear.

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Check also that the air mix door is operating properly, you could be getting outside air instead of heat. Then check your heater core if it is plugged then you wont get heat either, if the hoses both going in and out of it are warm at operating temp then it is good. Next (and I'm not sure if the 2000 models have it) but check the water cock at the heater core it is lever operated and controls flow through the core, it can rust shut easily.