I'm with labeledas on this -
There is some small liklihood of a math block affected by a conditional statement, but not likely The higher probability is a ground loop causing problems. A 4-20mA signal can 'ride' on common mode voltage.
Frequently the ground current adds to the mA current, then the ground loop drives the signal way off scale.
There are conditions where a handheld, battery powered milliampmeter will not 'see' the ground loop, because it is floating.
One way to test for ground loops is to put your meter on DCV and check from each signal lead to ground, then also check for ACV from each signal lead to ground. One expects to see the DC voltage that drives the current loop, but the (-) should be a low offset from zero. Common mode can come and go over time, depending on ground faults in the field.
Are the 'action pak' field modules isolated, so that a ground loop can't be created between the field device and the DCS
analog input card? You should get the spec sheets for those 'actionpaks' and confirm that each is isolated from input to
output and from its power supply.
DCS analog input cards vary in their ability to handle common mode. Any signal connected to a single ended, non-isolated
AI card can produce a common mode signal which affects any of the other signals as well. Are there other devices connected to this particular AI card?
I have never seen an isolated converter module fail so that the isolation failed and non-isolated signal passes through. Typically when they fail, they fail to pass any signal through. But a field wiring problem, like a unwanted connection to ground on the DCS side of the signal could be eliminating the common mode that the isolator is producing.