Diagnosis
This is the Auto Mode function. The AC5350B has a built-in particle sensor. When set to Auto mode (indicated by the 'A' icon on the display), the fan speed will automatically adjust between low, medium, and high based on the detected air quality.
How to Fix It
To lock the fan at a constant speed, press the 'Fan' button to cycle out of Auto mode. Select your desired manual speed: Low (one fan icon), Medium (two icons), or High (three icons). The display will show the selected speed icons and the 'A' icon will be off.
If That Didn't Work
If the fan speed fluctuates erratically in manual mode, the control panel or internal sensor may be faulty. Perform a hard reset: unplug the unit for 2 full minutes, then plug it back in. If the issue persists, the main control board likely needs service.
Heads Up
In Auto mode, the fan may ramp to high speed when you cook, dust, or have visitors, which is normal. Keep the particle sensor (located on the front panel) clean and unobstructed for accurate readings.