Low oil in sight glass

Initial oil charge was inadequate

Check at sight glass

Add oil

Compressor loses oil gradually

Oil settling out in system

Check pitch of suction and hot-gas lines for traps; run compressor continuously to return oil to compressor

Correct pipe pitch to remove traps; add oil if needed

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Dirty strainers and/or drier

Inspect suspect parts

Clean strainer, replace drier

Cold crankcase

Refrigerant flooding back and filling crankcase

Check crankcase heater with continuity meter, check TXV superheat; check contact of TXV sensing bulb to suction line

Correct problem found

Oil leaking outside compressor

Loose fitting and/or gasket; other leak

Inspect entire oil system

Tighten fittings and/or replace gasket or leaking part