R-134a is the refrigerant used in the majority of cars manufactured after 1993. Over time the refrigerant, also known as freon, can become low. When the freon is low the air conditioning system will not cool as quickly or even at all. Testing the air conditioning system with a R-134a gauge will help you quickly determine if your air conditioning system is low on freon. Reading the gauge correctly ensures you can accurately identify if the freon is low and is the cause of the air conditioning system not cooling properly.
Locate the air conditioning low-pressure service port on your car. Consult the vehicle manual if necessary. On most cars the low-pressure service port is near the air conditioning system under the hood. However, on some cars it is underneath the vehicle.
Remove the cap on the low-pressure service port by hand.
Attach the gauge to the low-pressure quick connect port by pulling back the ring on the gauge connector and pushing the connector in place.
Start the engine and set the air conditioning system to "Max A/C" and the system fan to "High." Let the air conditioning system run for two to three minutes.
Observe the R-134a psi reading on the pressure gauge. If it is less than 24 psi, your system is low and needs additional refrigerant. If your system is within 25 to 44 psi, it has the proper amount of freon. Readings of 45 to 64 psi indicate your system is overcharged and you need a mechanic to properly dispose of the excess freon. Any reading of 65 psi or higher is dangerous and your system is highly overcharged and might have other problems. Contact an air conditioning system mechanic.