Diagnosis
Timer countdown circuit malfunction. The most common cause is a software glitch in the microcontroller memory.
How to Fix It
Perform a full system reset. Unplug the fan from the wall outlet. Wait 2 full minutes. Plug it back in. Immediately set a timer for 1 hour. Do not use any other functions. Let the timer count down to zero to see if it powers off. If it works, the memory was cleared. If not, repeat but set timer for 15 minutes as a test.
If That Didn't Work
The tactile membrane switch for the Timer button may be sticking or failed, sending a constant signal that overrides the countdown. Listen for a distinct click when pressing the Timer button.
Heads Up
The timer function requires stable power to count. Using it with a mechanical outlet timer or on a circuit with brief power flickers can corrupt the countdown.