I do have an external sensor, which is on a separate cable per Honeywell's instructions.
also make sure that your sensor wires a twisted, however odd this seems, it makes a difference. I generally use 18/3 wire, stretch the length disired, and put it in you corless drill to twist it. Shielded wire can also be used, but not needed in all cases.
Very interesting! The installer definitely did not use a twisted pair ("homemade" or otherwise) in my install. What kind of bad effects do you observe if you don't twist it? I could understand seeing bad ODT readings, but do you also observe other effects on the behavior of the system?