LOGO! RTD Intermittent

I have Installed a LOGO with an RTD card in 2 brewery hot water tanks. We use rtds and the logo to control the electric heat. It worked great for several months and has since started to drop in and out randomly. If I isolate either one independently, they work. I have gone through many hours of troubleshooting, I will list:

- checked all RTD wiring and connections
- changed RTDs out
- ensured all grounding and bonding of tanks is adequate
- ensured RTD shield wiring is ground at only one end
- replaced RTD card
- isolated each RTD onto seperate cards
- ensured steady 24v supply to RTD cards

I'm sure I have tried several other things but I am out of ideas... Any thoughts?

Sounds like you checked the usual stuff..

Does someone around the brewery happen to use a handheld radio? Or does someone occasionally weld stuff close by?

No, its a microbrewery in a hotel. no welding, no radios.

When you say drop in and out, is that meaning the temperature drifts, or the unit shuts off?

Input just goes to 0. Then comes back, seems like a broken wire, but isn't.... Both sensors do it when they are both connected. Disconnect one and the other works fine.

Start smacking parts with the handle of your screwdriver and see if you can make it do it.

My first guess is that a floating ground, or power supply 0 V reference, is getting shorted to actual ground.

2 RTD’s, each on a separate input card go to zero and back intermittently.

Root problem is common to both RTD ’circuits’
”backplane” coonection for card-to-PLC?
card power supply?
??
Does the Logo run off a DC power supply? Same one as powers the RTD cards?

take out the sensors from the tank, grouding should be in the cabinet, not o sensor side.
check for correct grounding of the cards. (meaning one wire from the 24 Volt 0 to gnd, and the 3 sensor wires must be really sepaparte from ground.
Maybe a accidental contact from a sensor wire to gnd.