GE GEH Series Service Manual page 36

The T3a evaporator inlet sensor provides the control with
evaporator inlet temperature during sealed system operation.
The control checks the sensor to confirm:
1. Evaporator is frost free (above 32°F) at compressor startup.
2. Evaporator inlet is greater than 20°F after 30 minutes run time.
3. Temp difference between T3a and T3b is greater than 5°F after
30 minutes of run time
4. Using T5 ambient sensor, verifies T3a is at least 10°F less than
room temperature while sealed system is running.
5. The T3a, T3b and T5 sensors are all within 15°F of each other
before the compressor starts.
The sensor is considered open if the voltage at the board is
greater than 4.88VDC and shorted if less than .098VDC.
If any of the above tests fail, the control will stop the sealed
system and operate the water heater in electric mode.
36