Nissan Sentra 2008: Once I start the car, the AC comes at full blast and it won't turn off.

when the amp or in this cars case the blower motor resistor fails you'd have high speed but no low or medium speeds. the controls sometimes fail on these. is it only the blower speed that you cannot control or is it also the hot/cold or def/vent/floor controls that are acting up? I'd prob go to a dealer and spend a couple $$ as replacing the wrong parts adds up on these cars and cheaper to do it right first time.

You can change the resistor pretty easily. Go get one at your favorite parts store. It's under the glove box. You only have to remove the glove box (2 screws or a few more). You'll save more than half the price since the time to change the resistor is usually more expansive than the resistor itself.