When reviewing my annual logs, I noticed the PM 2.5 detector, an Ecowitt WH41 model, dropped off in readings in August, and was pretty consistently low for the rest of the year. I visited the WxForum and found an article suggesting that the air chamber could be clogged and needed regular clearing. Sure enough, I took the device apart and found debris in side, and what might have been a thin leaf stem stuck in the fan. Cleaned and blown out, the device returned to service, readings immediately started to show more interesting behavior. Hope that fixes it!
