Ventec
There are several reasons for this error
• Try another cassete to make sure it is the cassette and not the connector on the vent
• If you Autoclave the cassettes it could have being damaged internally ( they cannot be opened or repaired )or still have moisture inside let it dry
• I would advise against autoclave as per Maquet but you must then use their duo guard filter .This will give longer life to your cassette and less errors
• Replace the membrane and insure no dirt in there ( membranes should be replaced regularly
