What a trap is for, and how it stops working
A grease trap is a settlement chamber. Wastewater slows down inside it, fats and oils rise to the top, solids drop to the bottom, and the cleaner water in the middle leaves through the outlet. It has no moving parts and nothing to fail, which is why people forget it exists until it stops doing its job.
It stops working when the grease layer and the sludge layer grow far enough to meet the outlet. From that point the trap passes fats straight through into the drain line, and the problem moves from a tank you can service into a pipe run you cannot. That is the failure worth avoiding, because a blocked and coated drain line is a far larger job than a pump-out, and it usually announces itself during service.

