Diagnosis
The sanitize indicator requires the final rinse water to hit 150°F. If the light doesn't come on, the water isn't getting hot enough, or the high-limit thermostat isn't sensing it correctly.
How to Fix It
First, make sure you're using a cycle that includes a sanitize phase, like "Sanitize" or "Heavy Wash." Ensure your home's water heater is set to at least 120°F. Run hot water at the sink next to the dishwasher until it's fully hot before starting a cycle.
If That Didn't Work
The issue could be the heating element failing to bring the water to the required temp, or the high-limit thermostat (part of the heating element assembly, part #00654510) is faulty and not signaling the control board.
Heads Up
The "Sanitized" light is just an indicator. Dishes can still be clean even if the light doesn't illuminate, but it means the high-temp kill step wasn't verified.