You may be able to save a lot of time and money by trying to troubleshoot car problems yourself. Air conditioning problems are quite common, and might be caused by any number of things.
Start the car and let it run for a few minutes for the engine to heat up.
Adjust the climate controls on the dashboard to make sure they are correctly working. If the controls seem to work but have no effect on the air pressure or temperature then the problem is probably a blown fuse. If you cannot hear the accompanying blowing sound when adjusting the controls then it is probably a broken blower motor. If they are not working properly at all, then the problem is probably in the control panel.
Find the fuse box in a Nissan Sentra to the left of the steering column on the driver's side. Open the driver's door and pry off the side panel. There is a diagram on the inside of the fuse box cover that will indicate which fuses control the air conditioning.
Remove a busted blower motor by first removing the glove box and then the trim that surrounds it. Remove the three bolts holding the blower motor into place, and finally the blower motor.
Get to the control panel in a Nissan Sentra by first disconnecting both negative and positive cables to the battery unit. Remove the instrument panel by removing the two lower panel screws and the lower panel on each the driver's side and then the passenger side.
Unscrew the six steering column cover screws, the cover and combination switch, the two cluster lid "A" screws, the cluster lid "A", the three combination meter screws, and then disconnect the electrical harness connector and remove the combination meter. Remove the ashtray next. Remove the cluster lid "C" mask, screw and the cluster lid "C".
Take off the eight audio and air conditioning control panel screws, the electrical connectors and then the assembly itself. Remove the transmission shifter trim. Remove the rear console mask, the four screws and the rear console. Remove the four front console screws and the front console. Remove the front pillar garnish. Remove the lower dash side garnish. Remove the instrument panel mask. Remove the instrument nuts and bolts and finally the instrument panel.
