the auto mode light is on but the fan never ramps up even in a smoky room, seems broken.

Diagnosis

The Winix 5500-2 utilizes an optical particle sensor (the "Auto" sensor) that uses an infrared beam to detect the density of airborne particulates. When the Auto light is illuminated, the logic board is actively polling this sensor. If the fan fails to ramp up despite visible smoke or dust, the sensor has likely become "blinded." This happens when a layer of fine particulate matter or a physical obstruction (like a stray piece of lint or a spider web) settles directly onto the infrared emitter or the receiver lens inside the sensor chamber. Because the sensor is located behind a small intake slit on the front-right side of the chassis, it is prone to accumulating a "plug" of dust over time. When this happens, the sensor perceives the air as clean because the beam is blocked or diffused, preventing the unit from triggering the high-speed fan response. You can confirm this by manually switching the unit to "Turbo" mode; if the fan spins up normally in Turbo but remains idle in Auto despite a smoky environment, the issue is strictly with the sensor's ability to detect particulates, not a failure of the fan motor or the main PCB.

How to Fix It

To restore the sensitivity of the optical sensor, you must clear the internal chamber without scratching the delicate infrared lenses. Follow these steps precisely:

1. Disconnect the Winix 5500-2 from the wall power outlet to prevent any electrical shorts during cleaning.
2. Locate the sensor intake slit on the front right-hand side of the unit, just below the control panel.
3. Use a can of high-pressure compressed air. Hold the nozzle approximately 1 inch away from the slit and deliver three to five short, sharp bursts of air into the chamber. This dislodges the accumulated dust cakes from the lens.
4. Using a vacuum cleaner with a soft-bristle brush attachment, gently vacuum the exterior of the slit to remove any loosened debris that may have been pushed toward the surface.
5. Plug the unit back into the power source and press the "Auto" button so the indicator light is on.
6. Perform a "Sensitivity Test": Gently shake a dusty blanket or use a small amount of incense smoke directly in front of the sensor slit. The fan should ramp up to a higher speed within 5 to 10 seconds of detection.

If That Didn't Work

• Deep Clean the Sensor Chamber: If compressed air failed, the dust may be damp or "caked" onto the lens. Use a high-powered electric contact blower or a specialized vacuum with a crevice tool to create a stronger pressure differential across the sensor slit. Avoid using cotton swabs or needles, as these can scratch the IR lens or push debris deeper into the housing.
• Hard Reset the Logic Board: Sometimes the Auto-mode logic hangs. Unplug the unit and leave it disconnected for at least 30 minutes. This allows the capacitors on the main board to discharge fully, resetting the sensor calibration thresholds.
• Inspect the Pre-Filter: If the washable pre-filter (the outermost mesh) is completely clogged, air cannot reach the sensor slit efficiently. Remove the pre-filter and vacuum it thoroughly. If the sensor works while the pre-filter is removed but fails when it is installed, the filter is restricting the airflow required for the sensor to "sample" the room air.
• Sensor Component Failure: If the unit remains unresponsive to smoke after cleaning and resetting, the infrared LED or the phototransistor within the sensor module has likely failed. This requires replacing the sensor assembly, which is a separate modular component connected to the main board via a small wiring harness.

Heads Up

• Avoid Chemical Cleaners: Never spray glass cleaner, rubbing alcohol, or water into the sensor slit. Liquids can permanently fog the infrared lens or cause a short circuit on the sensor board.
• Regular Maintenance: To prevent future "blindness," use compressed air on the sensor every three months, regardless of whether the unit seems to be working.
• Placement Matters: Do not place the Winix 5500-2 against a wall or curtain where air stagnates. Ensure there is at least 12-18 inches of clearance around the sensor side to allow the unit to accurately sample the room's air quality.
• Filter Replacement: Always replace the True HEPA filter according to the indicator light; an overly saturated HEPA filter can cause the internal pressure to shift, occasionally affecting how air is drawn past the optical sensor.