Why does controlling power with a transistor interfere with MHZ19B's serial communication?
Just guessing but I suggest you use a level translator. Also when you power off the sensor the serial output should go to ground disabling that channel.  Check your voltages and put a bypass cap on the transistor for the sensor. You can use a simple P channel Logic Level fet,only a 50 ohm resistor in the gate to prevent oscillation.  Your output will now be active low instead of active high. If you stay with transistors make R2 &amp; R3 510 ohm so you have enough drive for the transistor and be sure it is NOT a darlington device, if it is you will not have enough voltage for the sensor.