my air purifier fan speed keeps changing on its own, i didn't touch the buttons

Diagnosis

The erratic fan speed behavior in the GermGuardian AC5350B is almost always caused by the unit being left in "Auto Mode." This model features an integrated infrared particle sensor located behind a small vent on the side of the chassis. This sensor constantly scans for airborne particulates, dust, and smoke. When the sensor detects a spike in pollutants—even something as simple as a gust of dust from a nearby curtain or a cooking aroma—it triggers the control board to ramp up the fan speed to High to accelerate the CADR (Clean Air Delivery Rate).

Once the sensor determines that the particulate count has dropped below a specific threshold, it will automatically step the fan down to Medium and then Low. If you see the "A" indicator illuminated on the control panel, the machine is operating in this dynamic loop. You can confirm this is the cause if the fan speed increases immediately after you vacuum the room or use a spray cleaner, as these activities kick up microscopic particles that the sensor is designed to detect.

How to Fix It

To disable the automatic adjustments and lock the AC5350B into a consistent, manual airflow speed, follow these exact steps:

1. Locate the Fan Speed button on the top control panel.
2. Press the button repeatedly to cycle through the settings. You will see the indicator lights change.
3. Watch for the "A" (Auto) light to turn off. As long as the "A" is lit, the sensor is in control.
4. Continue pressing the button until you reach your desired manual setting:
   • Low: One fan icon illuminated. Best for nighttime or quiet environments.
   • Medium: Two fan icons illuminated. Ideal for general daily maintenance.
   • High: Three fan icons illuminated. Use this for rapid air scrubbing or when odors are present.
5. Once you have selected a specific number of fan icons (and the "A" is dark), the fan will remain at that exact RPM regardless of air quality changes.

If That Didn't Work

If the fan continues to fluctuate even after you have manually selected a speed and disabled Auto Mode, try these advanced troubleshooting steps:

• Clean the Particle Sensor: Dust can accumulate on the sensor lens, causing "ghost" readings that trick the logic board into changing speeds. Use a can of compressed air or a vacuum with a brush attachment to gently clear the sensor intake vent on the side of the unit. If the sensor is clogged, it may send erratic signals to the motor controller.
• Perform a Hard Power Reset: Unplug the unit from the wall outlet entirely. While unplugged, press and hold the power button for 10 seconds to discharge any remaining electricity from the capacitors on the PCB. Leave it unplugged for at least 5 minutes before plugging it back into a different wall outlet (avoid using power strips or surge protectors for this test to rule out voltage drops).
• Inspect for Obstructions: Ensure the unit is at least 12-18 inches away from walls, curtains, or furniture. If the unit is too close to a wall, it can create a "pressure pocket" where air recirculates directly back into the sensor, causing the machine to think the air is dirtier than it is and triggering speed spikes.
• Check for Electrical Interference: If the unit is plugged into a circuit with a high-draw appliance (like a space heater or dehumidifier), electrical noise can occasionally interfere with the AC5350B's sensitive control board. Move the purifier to a dedicated outlet on a different circuit to see if the behavior persists.

Heads Up

Keep these maintenance tips in mind to ensure the AC5350B operates correctly:

• Filter Replacement: Replace the HEPA/Carbon filter every 6-8 months. A heavily loaded filter increases static pressure, which can cause the motor to work harder and potentially trigger erratic behavior in the fan controller.
• Avoid Aromatherapy: Do not use essential oil diffusers or incense directly next to the unit. The volatile organic compounds (VOCs) and oils can coat the particle sensor, rendering the Auto Mode permanently inaccurate.
• Vacuum the Pre-filter: Every two weeks, use a vacuum to remove large dust clumps from the exterior pre-filter. This prevents the motor from overheating, which can lead to automatic speed reductions to protect the circuitry.