john lewis jlfzw1806 only works on fast freeze

[madhousebill]

hi there guys,i hope someone can help with my problem,
my wife has just obtained a john lewis jlfzw1806 upright freezer but it will only work or stay frozen on fast freeze, i have checked the usual suspects as follows,

back of unit is clear from any dirt and dust.
if left off for 10 mins and switched on without fast freeze compressor runs for a sort time and then shuts down and never starts up again and the temp inside the freezer slowly rises.
if run with fast freeze it runs all the time and gets down to temp no problems.
checked all pipes on the back of unit just warm and checked pipes down near the compressor and they are not getting to hot.

this freezer has a air probe,evaporator probe and temperature limiter, not really sure what there for or how they work, i think found a link here:http://www.ransomspares.co.uk/parts/brands/john-lewis/fridges-freezers/jlfzw1806{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d}20(92234272000)/

im not sure if anyone knows if there is a way to test these probes or if the pcb has gone it would be really great if anyone one with more knowledge could help me on this one,

many thanks

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