Is your sensor comms meant to be 4800 baud? Most are 9600 baud. What does your user manual say?
What is the value of byteCount in your readModbusValue function just before it returns?
It looks like you are powering your sensor from the Arduino 5V. What is the lowest supply voltage your sensor is spec'd to run from?
By the way, these sensors don't work. Have a read of this very recent discussion where forum members with agricultural knowledge discuss it.