RS67A Freezer says it's -20C but actually only -5C

[ChrisChowings]

Looking for some help please.

The freezer part of my RS67A is not getting cold enough - it says it is -20C but it is only -5C when measured with a different  sensor. If you switch it off it will report the correct temperature(-5C) for a while before going back down to -20C whilst the actual temperature doesnt move.

I have:

Cleaned all grills on back of freezer

Removed evaporator cover, defrosted all ice

Checked fan working ok

Anyone else seen this before and managed to fix it? It's only 2 1/2 years old so don't want to have to think about replacing it.

Thanks

[ChrisChowings]

Update on the above.

Had a local refrigeration company to look at it, freezer has lost gas, probable internal gas leak and therefore un-repairable in their eyes... less than 2.5 years old.

Have contacted Samsung and awaiting an appointment from one of their approved engineers to assess. If it is un-economic to repair has anyone had any success claiming against the Consumer rights Act 2015 as <2.5 years use does not meet the "Reasonable Amount of Time" threshold.