Shower Defuser Plate 457-037-000 and Shower Screen 457-504-000 are held in place by the Group Head Counter-sink slotted screw 590-797-000 (though some times there is a Philips or Allen socket screw. Prevents theft). If the shower plate is stuck. It's a sure sign that back flush cleaning hasn't been done. Regulare cleaning is needed to prevent screen from clogging. This may cause "chanelling" where the water only flows though part of the coffee. Results in week, no flavor coffee.