Turn on the air conditioner and allow it to run for at least 10 minutes before testing.
When you're checking the low side of an air conditioner, you're measuring the pressure in its refrigerant suction line. This pressure tells you about how the compressor is drawing refrigerant and how quickly the refrigerant is evaporating (EVA) in the cooling coil. If the pressure is too high, then the EVA is too fast, which could mean that the compressor is damaged or the metering device is stuck open.
Set the manifold gauges beside the air conditioner. Ensure the low and high pressure gauges are closed.
Place the low pressure connector from the low pressure gauge over the low pressure port on the air conditioner. Turn the connector clockwise to secure it. These ports are located in different areas depending on the unit; consult your owner's manual if you can't find the ports or they're not labeled.
Place the high pressure connector from the high pressure gauge over the high pressure port on the air conditioner. Turn the connector clockwise to secure it.
Open the low pressure valve on the manifold gauges and then the high pressure valve.
Record the low pressure reading, which should be well below 90 psi (pounds per square inch). If it's above 90 psi, this could indicate that the refrigerant isn't evaporating properly.
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