[werk_ag]

Jim, also try changing the LCD address to 0x3F in the code.

Tried that no change. I think the unit has issues but  don't know where to start, still nothing on serial monitor
Regards
Jim

Hi Jim,

Even if the LCD is not working, you should have some output on the serial monitor.
Try revising all connections, start by having just the Temp / Hum sensor and the PMS7003

Werk good morning,
I have checked all connections (not that there are many) and all seem OK, and have tried with only the HTU21D and PMS7003 and still no result. I have even replaced the Nano with a new one.
I have a few observations which may give you a clue;
Input voltage measures 9.19v, regulator output 5.97v, the "L" led on the nano is constantly on, there is a regular red flash internally from the CO2 sensor and the fan in the PMS7003 never seems to move and i have not seen the Tx led flash.
I am at a loss!! 
Regards
JIm

Hi Jim,

The output voltage of the regulator is very high. I suggest that you quickly disconnect the sensors until you find why the output voltage at the regulator is so high.
Do you have a picture of the board?
Check if the TX module is installed in the correct position.