Some bars go off scale, either too high or too low
Might be in the Normalize view. Switch to Percentage view: hold LF button for three seconds.
Could be in Celsius but the alarm limits are still set for Fahrenheit.
If a bayonet or threaded adapter probe is installed, check that it is screwed in tightly and the threads are clean in order to ground the factory probe. If adapter is not grounded well to engine, voltage will be applied to the adapter thermocouple junction and disrupt the EDM instrument.