DIAGNOSIS The Sani Rinse indicator light only illuminates during a cycle if the final rinse water reached at least 155°F. If it never lights, it means the water heater in your house isn't supplying water hot enough, or the dishwasher's heating element or temperature sensor is faulty.
FIX First, check your home's water heater setting. It should be set to at least 120°F, but 140°F is better for dishwashers. Then, do a hot water test: run the hot water at the sink next to the dishwasher until it's scalding hot, then immediately start a 'Sani Rinse' cycle. If the light now comes on, your issue is incoming water temp.
IF THAT DIDN'T WORK The dishwasher's internal heating element might have failed. This element heats the water during the main wash and final rinse to reach the sanitizing temperature. You can test it for continuity with a multimeter, but it's a common failure point.
HEADS UP Even if the light doesn't come on, the dishes are still getting cleaned. 'Sani Rinse' is for sanitization, not cleaning. The cycle will still complete without it; it just means the high-temp kill step wasn't verified.