You have been to the job, these are ALL your observations?
I don't know your fridge, generic Q's below
Freezer -18°c in temperature.but the display says 31°c
Interior temperature of the freezer is the same as the set temperature,but the display says it was 31°F
Suggest use control panel to set to factory default, use 2 probes to monitor temp to assess/clarify display verses running condition.
Is the display flashing?
Check thermistors are in range against published values.
Does it have the perfect temp drawer, is it working correctly, and displaying correctly if fitted.
Is the icemaker working correctly if fitted.
Check door alarm working, check power failure alarm working.
Look for the cover as pictured