2004 Nissan Titan Heater

If it only gets hot while driving then you may be low on coolant and/or have an air pocket in your cooling system. When engine is cool remove the radiator cap, fill with coolant if needed, leave off the cap, start engine, turn on the heat, let the truck idle till it reaches operating temperature, you may need to raise the RPM to about 3000-3500 a few times to help push the air out of the system, do this a few times until heat at idle has occurred, and recheck coolant level then put the cap back on. Hope this helps.