Ck the coolant level, could need a thermostat,heater core could be blocked .
〉 Answered on Oct 28th, 2008 by Donna Burmeister, Service Manager at Classic Nissan
The heat that makes the cab of a vehicle warm is actually the heat that is constantly created by the engine. If you are not getting heat into the cab, it*s possible that the coolant is not circulating around the engine properly or that you have a blend door issue. The blend door is (usually a vacuum controlled) door that allows the heat from the engine to be routed to or away from the cab. To find out exactly what the problem is, I would recommend that you have it checked out by a trusted technician. Check out Ask Patty*s list of female friendly garages.
〉 Answered on Oct 28th, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management