A full GS3 reservoir can still produce “Refill Tank” if it is not contacting the rear level indicators. La Marzocco’s GS3 installation guide explicitly ties this message to that contact. For a reservoir-fed machine, check that the reservoir and drain box are fully back in their normal positions before assuming the pump has failed.
The drawer arrangement matters
The guide shows the reservoir sliding back into place after filling, followed by the drain box. Both need to sit correctly. Water visible inside a partly inserted reservoir does not tell you whether the tank is reaching those indicators.
Use the normal filling access described in the owner guide. Don’t bridge contacts or insert a tool to make the warning disappear. If the reservoir does not slide home normally, leave it for inspection rather than forcing the drawer into place.
First establish how this GS3 receives water
The same manual says the machine is configured for reservoir use and gives a separate procedure for connection to a mains supply and waste drain. Tell service which arrangement your GS3 actually uses. A report from a plumbed machine needs that detail before anyone applies reservoir instructions.
This is not a guide to changing the supply arrangement. Keep the existing plumbing connected and identify it in the service request. A tank-seating explanation should not become a reason to alter hoses or configuration settings.
The owner guide explains one documented reason for the message; it does not establish which component needs replacing on your machine. Save a photo of the displayed wording and note whether the warning first appeared after a refill. Those observations let the technician reproduce the complaint without guessing at a part.
Arrange GS3 diagnosis if the message stays. When booking, identify the machine as a GS3 and describe its use. The La Marzocco service page lists the home and commercial terms.
Source: La Marzocco GS3 Owner’s Manual, V2.0, November 2019, AV and MP installation guide: reservoir positioning.