I agree with amptramp's opinion, the EtherNet/IP is idle for future connection. You OIU communication with the PLC through the DeviceNet module. Nothing wrong with the E/I module.
Nothing wrong with the PLC system according to your description.
I suggust that: Check the linkage between Servo...
I agree with HEC. We need not to change the PH sensor. In most cases, just clean the sensor surface with water and re-calibrate the analyzer use standard solvent. The sensor will be OK.
use the total hydrocarbons analyzer, such as a FID analyzer,to measure the gas content. The gas content can show the relation with PH value of your gas stream, if it has the PH value.