If you put in refrigerant and later don't have it you either have a leak or a thief. If you have to bring service suggestions to your indy you need a new indy.

There could be other reasons why your compressor isn't engaged but the monovalve is NOT one of them. Fix your obvious issue before throwing parts at it.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician