DIAGNOSIS The laser particle sensor is either dirty, calibrated to a very high threshold, or has failed. It's giving a default 'good' reading because it can't get an accurate measurement.
FIX Clean the sensor. Unplug the unit, locate the small dark sensor window on the back, and gently wipe it with a dry microfiber cloth. Dust or a fingerprint can completely block the laser. After cleaning, plug it back in and let it run for at least 30 minutes in the room you're testing. The sensor takes time to sample and update the reading in the app.
IF THAT DIDN'T WORK Test the sensor's responsiveness. In the room with the purifier, create a safe, controlled burst of fine particles—like lightly tapping a chalkboard eraser or flour-dusted cloth from a distance. The 'Surge Mode' fan should ramp up within a minute or two if the sensor is working. If nothing happens, the sensor hardware itself may be defective.
HEADS UP The app's 'good'/'fair'/'poor' scale is based on Molekule's own algorithm, not a standard PM2.5 scale. It's less granular than a dedicated air quality monitor. Don't expect it to match a number from a different device perfectly, but it should definitely change when conditions change dramatically.