After a year or so of faithful work, I pulled in my DIY Airgradient kit to reprogram it to use MQTT instead of direct Prometheus support. And now I’m getting negative values for CO2 (-1, -2, and -3 respectively).
I touched up my solder work under the theory that a bad joint separated under the thermal and physical stress of being moved inside, but that hasn’t fixed it. Can these sensors just go bad after working for a while?