Liebherr Fridge Freezer

Hi,I have a Liebherr fridge freezer model Cnesf 3033 the top draw of the freeze defrosted, an engineer came out and replaced the defrost heater,removing the frost build up under the panel with a hair dryer. The freezer worked for 4 days then defrosted again. The engineer returned and suggested the fault lay in the “module” stating the machine to be beyond economic repair. The fridge works perfectly.I am an electrical and mechanical engineer but have no experience of refrigeration.Basic principles indicate that the fault must lie in the control system I am tempted to replace the PCB @ £115 but having read some of the previous posts I’m now wondering if it could be the temp sensors. can anyone help ?