Yogurt Curry Sauce problems, diagnosed

Look for curds versus oil. White grainy curds in a thin watery sauce mean the yogurt proteins coagulated. A smooth sauce with oil or ghee pooling on top is a fat-separation problem instead. Lower heat helps with true yogurt curdling, but it is not the only control: full-fat yogurt, tempering, and starch stabilizers such as chickpea flour can make a major difference.