Analog Output

Sounds like you have an alternate leakage path or the sensor is not calibrated. Your test of your cabling with a function generator may only prove continuity and not leakage. Calibration validation can get very complicated depending on your sensor.

You describe a classic 'ground loop' problem.

The analyzer is grounded at a different ground potential than the PLC, typical of all AC powered, '4 wire' devices. The analyzer analog output has a different ground potential than the PLC's analog input ground reference. Wherever there is a potential difference, current flows, in this case 0.5mA in the reverse direction.

The solution is to buy a "loop isolator" that take a 4-20mA input, isolates it, and retransmits it. An isolator is usually powered by 24Vdc. Connect the analyze r output to the isolator's input, connect the isolator's output to the PLC's input. If you have a 24Vdc power supply powering other stuff on the PLC side, use it.

I would not re-calibrate the analog output, because its output is 'correct'. The ground loop is altering it.

What is the transmit current at an intended 20mA <rhetorical question>? The resistance of the loop needs to be checked and if it is higher than the maximum stated in the manufacturer's instructions, steps must be taken either to increase the transmission voltage, reduce the loop resistance, install a retransmit device as stated above, or any combination of them.

How many other PLC's are connected in series with this output?Normally,a 250 ohm precision resistor is used as a load, and the resulting 1 to 5 voltage is actually used as the measured variable.

With each measuring device dropping 5 volts,at maximum, then 4 devices is the maximum number that can be driven from one transmitter with a 24 volt source.

I have seen larger systems where the resistor is 62.5 ohms,with a .25 to 1.25 volt output, allowing allowing more input devices to be series-ed.

I have also designed some custom systems to match some really weird, non-standard inputs, but I will not open that can of worms).

Go to the transmitter end of the signal cable,open the cable conductors with the PLC connected,and measure the resistance through the PLC input loop.It should be 250 ohms or slightly higher, considering loop wire resistance.

Check also for voltage on the line.It should be clean.

Make sure that the signal cable shield is only connected to ground at ONE END,not both ends

This bit of advice is in addition to checking for ground loops as mentioned by others here.Ultimately, isolators may be required.