DIAGNOSIS That's how the preheat function works on this model. It's a single-cycle timer. When preheat ends, it automatically switches to a default cook cycle (usually 5 minutes at the preheat temp).
FIX Do not use the 'Preheat' button for recipes. Instead, manually set your desired temperature and time, then let the fryer run empty for 2-3 minutes. Add your food, then press 'Power' to start the timer from your full set time.
IF THAT DIDN'T WORK If the timer starts counting down immediately when you set any time (not just after preheat), the control board's timing circuit is faulty. The unit needs professional service.
HEADS UP The preheat function is essentially useless for precise cooking. Ignore it. Manually preheat by setting temp/time and waiting, then adding food and starting the timer fresh.