2004 Nissan Maxima Blows cold air when idling

Could be a partially clogged heater core, bad thermostat, or air in the system, there is a tech bulliten about the correct level and bleed proccess for these cars, I will email a PDF to you. Check core and tstat like this: Heater Core "” Plugged

WARNING: The heater core inlet hose will become too hot to handle if the system is working correctly.

Check to see that the engine coolant is at the correct level.
Start the engine and turn on the heater.
When the engine coolant reaches operating temperature, feel the heater core inlet and outlet hose to see if they are hot.
If the inlet hose is not hot:

the thermostat is not working correctly.
If the outlet hose is not hot:

the heater core may have an air pocket.
The heater core may be restricted or plugged.
If both are hot, the temperature blend door in the heater plenum may be broken or stuck.