Hi,
My pre-soldered AirGradient DIY Pro (PCB v 4.2) has been working fine for several months, but, a couple of weeks ago, the CO2 reading changed to -3 and stayed like that ever since. No particular external event seemed to have caused this change; I simply checked the display at some point and the CO2 reading was -3.
Following the information in this thread Co2 reading of -3 constantly, I updated the AirGradient firmware to the latest version, but this did not resolve the issue.
The board itself was pre-soldered, and it was working completely fine for the first several months. Is it possible that a faulty S8 CO2 itself is the root cause here? Here is a picture of the current state of the board.