I recently had a customer who had an issue with a 4-20mA transducer input to an expansion I/O module. The problem was that the transducer's output was out of scale. For example, when it should have been sending 10mA it was sending 13mA, when it should have been sending 20mA is was sending 25mA. The problem was resolved by moving the input module's ground wire to the machine earth ground point where all the other ground wires were. So proper grounding is very important. I suggest making sure the grouning is done properly on all the components and controls.