If the air conditioning in your car is not pumping out cold air through the vents, you may need to recharge the refrigerant. Some cars have a warning light on the dash that indicates when the refrigerant level is low, while others rely on codes sent through the vehicle's on-board diagnostic (OBD-II) system. If your car was manufactured before 1995, then it may use R12 refrigerant, which is no longer being produced. However, you can have a mechanic retrofit your older A/C system so you can use an R134a recharge kit, which can be purchased at any auto supply store.
Locate the OBD-II port on your vehicle. It will be within 2 feet of the steering column and concealed behind a small panel that can be removed easily by hand. The OBD-II port is a 16-pin connector. Every vehicle built since 1996 is equipped with this on-board computer. Some vehicles produced in the early- to mid-1990s are also equipped with OBD-II.
Plug the scanner into the OBD-II port and turn the ignition key to the "Accessories" position. Turn on the OBD-II scanner on and wait for it to read the codes from the engine control unit (ECU).
Look up the codes reported by the OBD-II scanner by using a computer with Internet access to browse a table of error codes (see Resource #1). Other websites such as AutoZone also list this information in the online repair guide for the make and model of your car. The OBD-II code will indicate whether the A/C refrigerant pressure sensor is faulty or the refrigerant merely needs recharging. If you intend to replace the sensor yourself, remember that the OBD-II code does not necessarily indicate whether the sensor or the circuitry is causing the error that triggered the "Check Engine" light.
Use a multimeter to check the terminals on the electrical connection leading to the A/C refrigerant pressure sensor. Unclip the electrical connector leading to the sensor. With the ignition key turned on to the "Accessories" position, touch the probes to the terminals leading from the vehicle's electrical harness. If the multimeter does not register any voltage coming from the terminals, then your vehicle's electrical system rather than the sensor needs to be repaired.
If your car is equipped with OBD-I, you will need to look up information for the year and model of your vehicle for specific instructions, as numerous versions exist. Some OBD-I troubleshooting codes, such as on Chevys and Hondas, can be accessed by inserting a paper clip into two of the small holes on the OBD-I plug.