Same problem here. Picked up a new SHT30 online and it’s now working as expected.
I have this same problem, I ordered the diy pro kit and assembled it recently, right off the bat the temperature and humidity readings were zero. While building I removed the two pullup resistors from the SHT31 board as instructed. After finding this thread I tried installing some 2.2k resistors I had laying around and that got the temperature working most of the time but now the TVOC is stuck showing zero and occasionally the temperature still drops out. Today I saw the CO2 value drop a few times as well but so far the display is working fine. Did anyone ever find a conclusive solution to this? I’ve built quite a few gadgets around the ESP8266 and various I2C sensors and I’ve never had problems like this. I took a cursory glance with a scope right after I built it and saw reasonable looking activity on the I2C bus but it’s pretty hard to tell which data I’m looking at. In the meantime I ordered another SHT31 to try but I’m still waiting for that to arrive.
Try to update to the latest AirGradient Arduino library. We made the CO2 readings much more robust in the last update.