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How to test an AC pressure sensor

Updated March 21, 2017

Air conditioning pressure sensors, also known as AC switches, turn off a car's air conditioning system whenever the refrigerant pressure reaches a lower extreme. This not only signals the need for a recharge of refrigerant, but also saves the compressor from running without the proper lubrication. If the AC pressure switch is bypassed, then the compressor can potentially freeze up from a lack of fluid and lubrication, requiring expensive work to be done by a professional mechanic.

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  1. Turn your car's air conditioning system on with the engine running. Open the doors to the vehicle, preventing the air conditioning from cycling off. If the air conditioning cuts off intermittently, then the air conditioning pressure switch or sensor may not be working correctly.

  2. Open your car's bonnet with the air conditioner still running. Locate the evaporator, which is a block or grille-type component connected via hoses and tubes from the compressor, which has a belt and pulley system. Feel the two tubes leading from the evaporator to the firewall. If the tubes are not cold to the touch, then there is no refrigerant travelling through them, meaning that your refrigerant is either too low or the pressure sensor is not working correctly.

  3. Check the pressure of the air conditioning system by attaching a low pressure gauge from a refrigerant recharge kit to the low pressure fitting, located between the accumulator and the compressor and marked with a capital "L." According to AA1 Car, on an 80 degree day, the low pressure gauge should measure 56 psi (pounds per square inch) or higher. If the pressure is at an adequate level, then your pressure sensor may be failing.

  4. Locate the OBD II port on your vehicle, which is typically located within a few feet of the steering column. Many cars have the OBD II port below and to the left of the steering column, while other models have the port on the back side of the centre console.

  5. Plug the OBD II scanner into the port and turn on the car's ignition switch to the accessories position. Any sensor codes that come up on the OBD II scanner can either be read directly off of the scanner or you can look up the code on a website such as OBD II Code or AutoZone. If the air conditioning pressure sensor or switch is faulty, then the OBD II code will relay that information to you through the scanner.

  6. Remove the air conditioning pressure sensor and test the electrical connection leading to the sensor from your vehicle's electrical harness. Touch the probes of the multimeter to the electrical harness end of the electrical connection and turn the ignition key to the accessories position. If the sensor's electrical connection is working correctly, the multimeter should read between 4.0 to 5.0 volts.

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Things You'll Need

  • OBD II scanner

About the Author

David McGuffin

David McGuffin is a writer from Asheville, N.C. and began writing professionally in 2009. He has Bachelor of Arts degrees from the University of North Carolina, Asheville and Montreat College in history and music, and a Bachelor of Science in outdoor education. McGuffin is recognized as an Undergraduate Research Scholar for publishing original research on postmodern music theory and analysis.

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