Nissan doesn't use vacuum controls any more on the heaters, they use electric mode door actuators. There's one for the fresh air/recirc door, one for the air mix door (hot/cold) and another for the different modes (floor, defroster, dash vents). If it's auto climate control, the system should have self-diagnostics. The procedure, as well as symptom diagnostic chart for the HVAC system, should be in the factory control manual, probably in the "HA" section, IIRC. NICO Club's site allows free access to their online Nissan factory manuals, so it might be worth checking out.
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