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.
Use only Genuine Nissan Matic J ATF. Do not mix with other fluids. c Using automatic transmission fluid other than Genuine Nissan Matic J ATF will cause deterioration in driveability and automatic transmission durability, and may damage the automatic transmission, which is not covered by the NISSAN new vehicle limited warranty.