A couple of questions;
What engine version? W18V34SG?
What Engine control System?
Which exhaust thermocouple? ie. TE5011A Exhaust temp after cylinder
Minor clarification a thermocouple should produce a millivolt signal not sure that the resistance measurement would give any meaningful information. Have you tested the thermocouple utilizing a thermocouple meter to see if the issue is either the thermocouple or the engine controls? On the W20V34SG machines these are a type K thermocouple. I have seen some issues with WECS-8000 CCMs from time to time, typically all the TCs on the CCM go bad at once and normally it is a CCM ready to fail. Thankfully we have replaced very few engine cylinder exhaust TCs to date.