NodeMCU v2 with MH-Z19B CO2 sensor UART read problem
It says in your description that you have the Rx connected to Rx and the Tx connected to Tx.  That's backwards.  What one transmits the other needs to receive.  When you have a conversation with someone, do you talk into their mouth and listen to their ears?  You should have Rx-Tx and Tx-Rx.  

You also need to be sure that it is OK to feed a 5V Tx into that Rx pin.  On some ESP8266 boards it is and on most it isn't.  If you don't have one of the boards that specifically has circuitry built in to handle 5V inputs then you will fry your NodeMCU