The output of the transmitter is fixed at 20.8 mA, which shows  that the variables of the current main process is greater than the upper limit of range set at the sensor, and the instrument is in saturation state of the output The following items can be checked: 

1. Check whether the upper limit of the set range of the sensor is greater than or equal to the current signal to be measured, and determine the correctness of the model of the selected sensor and the correctness of the set range. 

2. Check whether the pressure-guiding pipe leaks or is blocked, if the pressure-guiding valve is used, check whether the check valve is fully opened or not. 

3. Confirm the incoming signal to be measured is the stable input. If it is liquid to be measured, confirm there is no residual gas in it; If it is dry gas to be measured, confirm there is no liquid. 

4. Check whether the sensor flange has precipitation or not and whether the flange has corrosion phenomena or not. 

5. If it is remote flange- type transducer, check whether there is a difference between two signals to be measured, and calculate whether the differential pressure caused  by the potential difference is greater than the sensor range or not. 

6. Check whether the power supply is between 12 v ~ 24 VDC or not. 

7. Use the hand-held operator to self-inspect and read the parameters of instrument and check whether the intelligent electronic components are in failure or have not been initialized.