When the compressor is not making air the standby pressure (inside the compressor) must be about 3 bars. You must mount a gauge at one of the pressure ports and check that if the compressor is turning and the compressor is set to off (electric switch) then the pressure must be about 3 bars. If the pressure is not correct you must locate the adjusting screw (you must remove the air filter) in the air inlet and adjust.
Notice: You must open the valve for the air system so you are sure that the compressor will not release air (that the minimum pressure switch is closed).
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