We have a bosch kgn34x60gb fridge freezer. The fan that circulates the cold air from the freezer into the fridge compartment runs virtually constantly. The freezer is set at -18deg and the fridge at "3". Both compartments have contents but are not jam packed. It seems that it should not need to run so much. The constant noise is unwelcome. How much should it run?

Call them out under warranty. the circulation fan is normally at the back of the fridge. On such a new fridge, so shouldn't need to be fixing issues. A fan is an 'electrical component', not an electronic item.

https://www.google.com/search?q=Bosch+Series+2+KGN34NWEAG+Fridge+Freezer+warranty

Refrigerators

• Limited warranty, entire appliance, parts and labor (1 year).
• Limited warranty for non-electronic components; parts only (1st to 2nd year).
• Limited warranty for electronic components; parts only (2nd to 5th year).
• Limited warranty for rust-through on stainless steel; parts only (lifetime).

Take the cover off the fridge fan and put a flash light in there and a camera set to record. close the door for a couple of minutes and then look at the footage to see if the fan is running. The circulation fan should run all the time.

Typically that'd be where they are..you should see the openings in the back of the freezer and the fridge...barring any obstructions sounds like a computer board issue

See or hear if the fan inside is working. Im not conversant with your model. Some units have 2 fans , 1 in the freezer and 1 in the fridge. Some only have1 , which will be in the freezer only. If the fan fails, the fridge cannot blow cold air into the food compartment. Generally, when an evaporator fan fails, the fridge doesn't get cold and the freezer overfreezes causing an excess ice buildup in the freezer.

Most of the time it runs only when the compressor is running. In other words compressor and fans will run together when the fridge thermostat ask for cold and stops together when thermostat is satisfied. In newer fridges some time inside fan (evaporator fan) might PERIODICALLY run without compressor to circulate little cool air to fridge compartment from freezer compartment.

Can you feel air moving or hear the fan running in the freezer section? You said the freezer section is freezing well, but is it freezing really hard?
Regards,Richard

All your cold air for the fridges comes from your freezer compartment.If you are haveing ice build up in the freezer comparment there may be an issue with your defrost circuit.

While the unit is in a cooling mode open the freezer door and listen to for the fan running in the freezer this is the evaporator fan that circulates the cold air to your fridge compartment if it is not running replace it.

If it does run try and check your defrost circuit and make sure the unit goes into the defrosat cycle

check the damper door located between the freezer and refrigerator compartments(this lets in the cold air from the freezer) and make sure it opens and closes with your temp adjustment in the refrigerator compartment,sometimes they ice-up and are frozen in one position then take a hair dryer and defrost the door

 If the evaporator fan is not running. The fan itself could be faulty or the door operated light/fan switch could be faulty.

If the evaporator fan is running. The fridge is not defrosting and ice has built up on the evaporator and the air is not circulating properly into the fridge compartment. The fan circulates cold air into the fridge and back through the fridge control which is normally just an adjustable flap which allows more or less cold into the fridge depending on the setting. If the fridge is not defrosting, the defrost timer could be faulty or heater element could be blown.