Your plant has a process that uses treated water. Based on the flow rate to the process, the PLC program calls for makeup water to be delivered to the day tank. Recently, the day tank has repeatedly run dry during production. The lack of treated water ruins the product. It also forces production operators to shut down, perform purges and blowdowns, and then restart the system.
Preliminary troubleshooting has established the input side of the control loop is problem-free. In the notes, you read:
First, examine the day tank. Is there evidence of a leak?
Next, determine if the control loop input side really isn't the source of the problem. This is somewhat simplified now, because the flow meter has been calibrated. Simulate the flow transmitter signal and watch the PLC for the corresponding changes. If it checks out, you need to focus on that output side.
Put a dummy load on the analog output module and force the PLC output at 10%, 25%, 50%, 75%, and 90%. If the module output tracks, repeat this but now stroke the makeup valve from the PLC (force). If the valve strokes properly, you probably have a piping problem. You'll need to arrange for hydrostatic (to check for piping integrity) and dynamic testing on the piping.