DIAGNOSIS Normal buildup. The disposable foil grease tray liner is either missing, overflowing, or not installed correctly. The catch pan underneath can also fill up.
FIX Pull out the entire grease management system from the front of the grill. Remove the used foil liner from the metal tray. Wipe out the metal tray with a paper towel. Line it with a new Weber 7.99 x 10.75 inch disposable foil tray (or equivalent). Also check the catch pan—the black plastic bucket—and empty it. Slide the whole assembly back in.
IF THAT DIDN'T WORK If grease is dripping from elsewhere, check the flavorizer bars for heavy buildup. Grease can drip off them and miss the tray. Clean the bars.
HEADS UP Never let the grease tray or catch pan fill completely. It's a major fire hazard. Replace the foil liner regularly.