There is a mode door motor that controls; Fresh/Recirculated air, Bi-Level, Blend Air, Heat, Defrost and vetilation.
So, I think this is your problem.
Here are the instructions for installing a mode door motor.
Installation of Mode Door
1. Rotate the side link by hand and hold mode door in vent position. Install mode door motor and connect wiring harness connector. Turn ignition on. Turn, (Depress) vent switch on.
2. Install rod of mode door motor to side link rod holder. Turn, (Depress), defrost switch on. Ensure side link operates at fully open position. Also, turn, (Depress, vent switch on to ensure side link operates at fully open position.
It is under the dash near the heater core box and connector comes from control panel. You should check it using the operation checks from the installation instructions before you buy one to make sure its not something you can fix.
As far as the heat not working well. If the car always did this Altimas are notorious for not holding heat well and the heater temperature flucuating as a result.
The other control for the heat works the blend air door and the heater control valve. It is an interesting design as the heater core has a constant flow of anti-freeze running through it. The cold air gets vented around it. So, the heater core is always helping to cool the car and the flow of anti-freze keeps it in better condition.
Here are the instructions for the Installation of the Blend Air Door and the Heater Control Valve. It is the only diagnostics I have avaiable as all you can do is follow the installation, as I mentioned earlier to go through the checks for operations in the instructions before you buy any parts, and is the only check for system available.
The Temperature lever
This lever setting positions the blend-air door to direct flow of air though the heater core, (hot setting), around the heater core, (cold setting), or a mixture of both. The lever also controls opening and closing the heater valve. At any setting, except cold, heater valve is open, allowing engine coolant into heater.
Here is the installation instructions for the other components in the heater controls
1. Move the blend air link by hand and hold blend air door in full cold position. Install blend air door motor on heater unit and connect wiring harness connector. Turn ignition on.
2. Slide the temperature control lever to full cold position. Attach heater control valve to blend air door linkage and secure with clip. Rotate heater control valve lever and plate in full, (clockwise) cold, position.
3. Ensure temperature control lever is in full cold position. Attach heater control valve to blend door linkage and secure with clip. Rotate heater control valve lever and plate in full, (clockwise) cold position.
4. Attach heater control cable to plate and secure with clip. White mark on cable housing should be centered under clip. Ensure heater control valve operates properly. After several cycles, heater control valve lever should be midpoint of plate opening when temperature slider is full cold.
Come back if you have any questions.
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Tuesday, September 27th, 2011 AT 12:10 AM