Hi, i have this fridge, i have replaced the thermostat but it still reaches down to -40 sometimes!!
using a household thermostat the customer placed in it reads pretty much the same so definately getting too cold, even tho there is no excessive build up of ice any where!!
Either you have not connected the stat to the correct position on the evaperator plate or the gauge that the customer is reading is duffed..the latter is my guess as it wouldnt reach -40 evan if it was constantly running for weeks on end.