Ok, so I've been pulling my hair out over this issue all day and I cannot for the life of me find the solution.
I am running an Uno which has 4 sensors connected to it, 3 DHT22 temp/humidity sensors and 1 Senseair K22 I2C CO2 sensor. The Uno is being powered off of 12vdc wall adapter and the sensors are powered/grounded off the Uno 5v. I used 1 cat5 cable, about 13' worth to run 3 separate wires for the DHT22 signals, 2 wires for I2C signal and then used a common power and ground wire for all 4 sensors. I ran the two common supply wires to the K22 CO2 sensor, soldered them to the board along with 3 additional supply wires for the DHT22's. So power and ground for DHT22's splits off from the K22 board.
The problem I am running in to is when I have the Uno connected via USB, the K22 gives stable read outs. When I remove the USB connection the K22 begins to give erratic readings. The DHT22's remain unaffected, however. If I simply touch the outside of the USB connector to the outside of the USB port on the Uno the K22 begins to send back stable readings. This leads me to believe that I have a ground issue.
I'm also running a 4 brick 5v relay board off the Uno and that works no problems... Everything seems to work fine without the USB ground being involved other than the K22. I'm starting to wonder if it doesn't have to do with tying pwr/gnd of all 4 sensors together the way I did?