Thermocouples work with millivolt signals which are inherently susceptible to the influence of induced noise from higher voltage leads that are near them.

Where thermocouple sensors are required, I try to use signal conditioners mounted as near the thermocouples as possible to convert those signals to analog current (4 to 20mA) which are mush less susceptible to noise.

Analog current PLC input modules are less expensive and often available with a higher channel density and can accept signals from a multitude of other types of sensors, so they're much more versatile as well.

Here's an example of a signal conditioner that is designed to fit into a thermocouple connection head:

https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/temperature_sensors_-a-_transmitters/temperature_transmitters/xth2-n100100f-t

Is this a problem that arose after working well for some period of time?

If so, how long did it work fine?

Is the thermocouple wire routed nearby any VFD power input or output wiring?

I agree with evrything OkiePC listed about the thermocouples. I also recommend the 4-20ma thermocouple transmitters for all the reasons he listed. You say "We get no issues when running anything else only this one single drive". Anything different about this one drive, T/C, wiring, cabling, etc.? VFD RFI jumper connected/ disconnected? T/C grounded/ ungrounded? T/C cable shielded/ unshielded? Grounded/ ungrounded? T/C cable routing close to motor/ motor power cable? Motor power cable shielded/ unshielded? Grounded/ ungrounded? T/C commons connected/ disconnected? to power supply common?

Yes, we have seen jumping T/C readings. The solutions can vary depending on the problem. For isolated, individual problems (it sounds like you have this) we have tried to trouble-shoot, as suggested above. For "normal" noise, we have used PLC filtering. For bad environments we cannot control, and for the reasons OkiePC mentioned, we have used 4-20ma temperature transmitters.

I would use a transmitter but we normally use temperature less that -100 deg F. We would be looking for something that would have a range of at least -250 deg F. Type T thermocouple.

What we have found is that the initial issue of the temperature reading too, low was caused by the noise also.

This is what we have found this morning.

1. With all thermocouples removed and the THM shorted out it reads fairly accurate. (ambient temp of module)
2. With one of the thermocouple connected but not touching the cabinet it reads fine. Once it touches the cabinet then there is an issue. So we think it is noise
3. Even with all of the drives powered down ( 460V removed from the drives) the issue remains. We will remove all 460V voltage from the machine to see if it helps. Remove the transformer from the system also.

The VFD is exactly the same setup as the remainder of the VFDs. We tried swapping out he VFD with now luck.

I really like using the programmable universal transmitters, like the SCU-1400 for example, and they exceed your requirements for -250 deg F, type T T/C. But they require mounting on a DIN rail in an enclosure.

"Once it touches the cabinet then there is an issue". Sounds more like a voltage issue, a difference in ground potentials or "stray" voltage. We have had "stray" voltage problems where the VFD-driven motor was inducing (high frequency) voltage and current was flowing thru the shaft bearings, and destroying the shaft bearings in a matter of months. The solution (as directed by the "stray" voltage consultant) was to install bonding jumpers everywhere, including installing carbon brushes riding on the shafts.

"Even with all of the drives powered down ( 460V removed from the drives) the issue remains." So not induced by the VFD carrier frequency. But a difference in ground potentials or "stray" voltage from somewhere.

I am guessing these are grounded T/Cs (T/C junction connected to probe). Can you try an ungrounded T/C (T/C junction isolated from probe)? Can you insulate the entire T/C probe and cable from the cabinet? (And still be able to measure temperature)

This is what we have found.

We took the panel completely out of the machine and started there. It turns out it may be the ethernet cord or the ethernet port on the PLC. I am curious as to if any one has seen the issue with the ethernet port on the PLC? If we plug a cable into the port and move it around it (the temperature) will fail. If we leave it alone then everything seems to work. We have a CAT 6 ethernet cord coming we were using a CAT 5 ethernet cord. So we are hoping that is the issue.