Couple of questions:
What type of temperature sensor? There are three common types. RTD, Thermocouple, and thermistor.
Resistance Temperature Detectors are basically a length of wire that changes resistance with temperature. Usually come as 2 or 3 wire versions. Very linear over a wide temperature range.
Thermocouples are two different wires that produce a voltage when one end is hot and the other end is cold. They come in several types, usually color coded, and are polarity sensitive. Fairly linear and most commonly used in process control applications like this.
Thermistors change resistance significantly however are useful over a narrow temperature band.
All three sensors require wiz bang electronics to get a useful signal out of them.
Does the temperature sensor connection have polarity or color markings on it? If so, that probably means it is set up for a thermocouple. Try swapping the leads and see if the temperature reading changes. Verify the color code on the replacement sensor wires matches the original color code.
Verify that a sensor lead has not been swapped with a shielding connection.
Good luck.
