The error E2 appears when the controler detects a problem with the the settings stored in memory. It indicates that either the settings are stored improperly or that the memory has been damaged. To solve the first possible cause, simply check the settings. If the E2 error persists after the settings are checked then the memory is damaged and the unit must be replaced. The manufacturer does not offer any replacement parts for the ETC controllers except for the temperature sensor.
It is possible that the E2 error shows up consistently at certain temperature or differential settings. This is an indication that only a portion of the memory may be bad. It is recommended in this case to replace the controller.
There is no specific cause for this error. However, In our experience voltage spikes may be associated with the error as can heat in excess of the manufacturers recommended ratings. You may wish to double check the location of the controller so that it is not located where temperatures are too high.