Diagnosis
The upper heating element is failing or the temperature sensor is giving a false high reading, causing the unit to under-heat.
How to Fix It
Run a test. Set it to Air Fry at 400°F empty for 5 minutes. Carefully feel the heat from the top vent. It should be very hot. If it's warm or cool, the upper quartz heating element has likely failed.
If That Didn't Work
If the element glows red but food still doesn't cook, the temperature sensor (thermistor) is faulty and telling the board it's hotter than it is.
Heads Up
Do not operate with a suspected bad heating element for long. It can cause other components to fail trying to compensate.