“How to Scan I2C Address in Arduino - Hackster.io：https://www.hackster.io/abdularbi17/how-to-scan-i2c-address-in-arduino-eaadda”

this is very helpful to me. I just followed your instruction and it seems no problem with connection. 

Is it normal to get 0.0 ppm from both sensors inside the office in general?

Hi! 

As shown in the picture above, I purchased three sensors, sen0470 , sen0467, and sen0231, and connected each sensor to Arduino UNO according to your online instructions.There is only a message "No Device!" message. Can you give any advice?