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Parts for Frigidaire LFFH21F7HWG Freezer - AppliancePartsPros.com

Thermistor Error Mode: Alternating Flashing Display of E8/Temp or E9/Temp
Upon entering the Thermistor Error mode and an open thermistor condition is present, the display will alternately display an “E9” for 0.5 seconds and “-10°F”
for 0.5 seconds (-23°C if in Celsius mode).
If a shorted thermistor condition is present, the display will alternately display an “E8” for 0.5 seconds and “-10°F” for 0.5 seconds (-23°C if in Celsius mode).

The thermistor is at the bottom of the lisying of Section 07.

Could be a bad thermistor.

Also may be a bad control board.
If unplugging the unit for a minute or so resets everything then I would lean towards the control board.

Also check that the evaporator freezer fan runs OK.