How to use A/C manifold gauges

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An A/C system in your car that is blowing hot air instead of cold air is major inconvenience. Pinpointing the possible cause of the A/C malfunction requires the use of an A/C manifold gauge and some interpretation of the gauge readings. Some corrective action steps can be taken by the do-it-yourselfer, though other corrections may require the services of a professional auto air conditioning specialist.

Skill level:
Moderately Challenging

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  1. 1

    Connect the low side hose (blue) to the low side port on the A/C unit.

  2. 2

    Connect the high side hose (red) to the high side port on the A/C unit.

  3. 3

    Open both valves so that the gauges register the A/C system pressure.

  4. 4

    Compare the actual PSI readings to the normal PSI readings with the A/C system in the "off" position (static pressure). The normal reading on both the high side and the low side should be 80 to 105 PSI.

  5. 5

    Turn the A/C system on to "max/high" output. With the engine speed at about 1,000 RPMs, check the low side PSI reading, which should be about 25 to 35 PSI, then check the high side PSI reading, which should be about 200 to 350 PSI. If the PSI readings are lower than normal, hook up a can of refrigerant to the yellow hose and add enough refrigerant to the system to bring the PSI readings up to the normal ranges.

Tips and warnings

  • The low and high hoses have different size connectors.
  • The owner's manual may have specific PSI ranges for your car's make and model.
  • If the PSI readings are normal and the system is still blowing hot air, you may need to consult an auto A/C professional.
  • Do not add refrigerant to the system without first using the manifold gauges to check the refrigerant level.
  • Do not overfill the system with refrigerant. Excess refrigerant must be removed by a professional and exhausted into a proper container.

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