You can't have interference on the temp sensor.
The way the unit measure temperature is by reading a variable resistor.
The only way to change the value is by changing the value of the resistor.
If you pinch the cable and short it inside or damage the temp sensor.
Or if you have an external temp sensor, you could look into the connector
for debris or corrosion (doubt it since plugs are gold plated)
Since you have a 798, look into the cable connector (quick mount) to make
sure everything is fine there.