Truck will not switch ac to heat is it the switch or something else

The mode selector is bad. IF you look at the plenum (where the fan motor is ) you should see two levers one is hot cold the other is mode. Check them by moving the controls and see if they move.
If the mode lever moves then the door on the inside is broken or something could have fallen in the vents and is blocking it.

Could be a heat sensor or the fan itself

Can you hear the front ac/heater fan? They use the same AC compressor so that should be okay. The fan switches fail quite often on those. That controls how much air is pushed by the fan for AC, heat and defrost. The switch is attached to the fan knob on the dash.

Disconnect the electrical connector to the AC compressor clutch and try again. If the fuse doesn't blow, then you've got a shorted AC clutch coil.

what switches did you replace. sounds like you blend door motor is stuck

Sounds like a vacuum activated switch. Your engine vacuum is used to open and close little doors in your vents. If a hose is loose or disconnected or the electronic controls that tell the switch to let vacuum thru is not working, no air will reach your defroster vent at the windshield. Does the switch that controls Vent, Heat (sends to the floor), AC and MAX AC do anything at all? If they do, you may have a specific switch problem. If you switch from Vent to MAX AC the sound of the air flow should be much louder. If the switch isnt changing where the air blows, it is leading you to the problem. Either the switch is bad or the supply of vacuum to the switch is not there whether it is a loose vacuum hose or a disconnected one. Happens often when aftermarket stereos are installed and the installer disconnects something either by accident or forgets to reconnect it.

Your problem could be in your control switches or a bad heating element. Using a voltmeter check for 240 AC volts at the heating element connection. If voltage is present your heating element is most likely the problem. If no voltage is present then something in the control circuit is malfunctioned. I had this same problem about a month ago. Using the above troubleshooting I determined the heating element was defective.

It could be worn contacts on the HVAC selector switch. It could also be a bad blower motor but I would check the switch first; contacts lose continuity from a lot of use and wear down. both should be able to be had from a junkyard for rather cheap. I would suggest a loose connection but I don't think that is the case here unless the problem were intermittent.

Good luck!!

I have a 2002 nissan Maxima. The AC controls only blow on level 4. No air or heat blows from level 1-3. What do I need to do to repair this problem?

you need a new blower motor