Cheese Sauce problems, diagnosed

A split cheese sauce has lost its emulsion: fat separates from the water phase, often leaving visible oil along with grainy or clumped proteins. Visible oil is the key clue that separates a split sauce from one that is merely gritty.

A grainy cheese sauce can be an overheated protein problem, a broken emulsion, a roux problem, or a reheating problem. First check whether the sauce is also oily and when the graininess appeared.