check for corrosion on connections of wires to rtd probe. check position of probe in plenum. since the probe is an averaging type, the greater amount of actual probe in the plenum, the better the reading. do not change the probe unless the amount of error is a large amount (+,- 100F) these probes are highly accurate and the normal failure mode is an open circuit

on full heat "sd" models, the probe has to be mounted in the center of the plenum. (vertically) if it is mounted near the floor, the readings will be too low because most of the heat is near the top of the plenum!

Check to make sure that the shield wires for both lines running from the Slave Board to the AC Drive are properly grounded. If the reading jumps to 352F or 29F and then returns to the proper value, check for a short or an open circuit in the wires running to the probe. It is also possible that the probe itself has an intermittent open circuit internally, which would cause the reading to jump high and then return ti its normal value. If the reading is swinging only below its normal value, the problem is either noise, or there is a short-circuit in the wires running ti the probe. Under this condition, the probe is not defective. If the reading is swinging above and below its normal value, the problem is probably noise generated from the AC Drive.