Air Vents not working

[Jigga412]

I've got a problem with my 99 Sentra. When I turn on the fan to use the heater/ac/defrost no air blows out. It is only when I turn it on full blast that air will come out. I don't mind doing this every so often- but full blasting air can get pretty loud! Any ideas? I've tried looking at the fuses and all looks good. Some have suggested looking at the 'relay'- where is that found? Is this something I can fix myself?

[yogi_123rd]

I believe what you are really saying is that the low and medium blower speeds don't work, but the high blower speed does.

Most blower systems have two circuits - one for high speed and one for low and medium speeds.
I am not familiar with the sentra setup, but I suspect the basic setup is the same as on most cars.
Basic setup:
There is a blower motor resistor which is mounted in the blower box. It's purpose is to reduce the voltage in the circuit going to the motor to give low and medium speeds. Some cars it is accessed from the engine firewall and on others it will be on the inside where the heater is. In all cases, it is mounted in the blower box. The device is just coils of wire that looks like springs (it is a mini heating coil) and it's purpose is to reduce voltage. These blower resistors fail when they burn or corrode thru the coils breaking your circuit. You then lose the low and medium speeds. That is your first possible problem.
There is also a relay involved. Both the low speed and high speed circuits go thru this relay. If you can locate this relay, you will note that some of the wires go to the blower resistor and one of the wires go to the blower motor. If you disconnect this relay, all blower speeds (hi or low) won't operate.
Usually the high speed circuit consists of a hard hot wire. The low speed circuit is the output from the blower resistor. When high speed is selected on the heater control (and ignition is on), the relay activates switching the power source to the hard hot wired. The second possibility is that this relay is bad. The low speed contacts in this relay can be burnt or pitted.