Are all the thermocouples showing an error or only the ones immersed in water? Are you connecting the thermocouples directly to the PLC? If you are using more wires as extensions, you will be creating more junctions and you will definitely see an error. Also, make sure only the tip of the thermocouple is immersed in water and not the insulation. We use thermocouples in our units all the time and I have found it to be better to linearize the output from the thermocouple using reference temperature at least 30 deg C apart rather than using the thermocouple output directly.