Liebherr KB4250 auto defrost failure

[jag157]

Liebherr larder fridge KB4250-21F Service No: 9983182-06

I possess the following appliance service leaflet: Service Information No 25/2004, KB(es,a) 3650/4250

For several months the fridge automatic defrost cycle has failed about every 4 days. This is evident because the water level in the evaporator tray falls quickly, and the defrost drain tube becomes persistently dry. If no action is taken the rear wall evaporator ices up, initially reducing its cooling efficiency but eventually blocking the airflow so that the fridge becomes unable to maintain either the set (and digitally indicated) temperature or the correct Biofresh compartment temperature. (From previous experience, for food safety reasons I keep 3 spirit thermometers in the appliance.)

When the auto defrost fails there are no error F codes displayed; selecting the service menu shows normal indications in CS mode “L”, and in sensor test mode “E” all sensor temperatures are as expected. If I then reset the appliance by disconnecting power for 30 minutes the auto defrost cycle is restored temporarily and the evaporator tray is soon topped up with melt water.

The need for this abnormal defrost management is merely inconvenient while I am at home. But if I am away for several weeks the fridge temperatures rise significantly. Then on return, and after a much delayed reset, the evaporator tray would overflow unless emptied, and melting ice lumps can clearly be heard falling from the rear wall evaporator.

Seven years ago the fridge iced up and the auto defrost could not be reset. When I replaced the evaporator sensor the fridge then performed normally until 6 months ago. However, the current symptoms and the response of the system to resetting lead me to suspect that this latest fault might perhaps lie in the electronic control module.

I would welcome your diagnosis and advice for rectifying this fault.

[Tony R]

You need to check the temperature at the sensor bulbs and confirm they are reading correctly by checking the resistance corresponds with the bulb temperature, checking in the E menu only shows what the sensor thinks what the temperature is.

[Author]

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