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I have a Bosch KGH34X50GB fridge freezer, usually set to -22

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I have a Bosch KGH34X50GB fridge freezer, usually set to -22 (it seems to reliably maintain the freezer temperature) and the fridge compartment keeps getting warm - on investigation I've found that this is partially due to ice forming in the small channel between the freezer/fan and the floor of the fridge, but the fan can be heard working (and air can be felt coming from the fridge vent when the channels aren't blocked, though I think it still struggles to get the fridge temp down low enough even when defrosted), the thermostatic air valve in the rear air channel opens and closes in response to temperature change, the heater element below the evaporator is working (it can be felt getting warm when it goes into a defrost cycle), water is passing down to the evaporation tray and there don't appear to be any seals etc. leaking, and I've tried manually defrosting twice - what can I be missing that's causing it to ice up constantly? I've verified that the fan spins freely too - the only thing I've noticed is that the circulation fan doesn't run constantly - one thing is specifically like to know for this particular model is whether there is a temperature sensor for the fridge controlling the operation of the fan (I can't physically locate one), and/or whether the fan should run constantly (my thinking is that perhaps the control PCB is faulty and not running the fan enough - I have read that Bosch changed fitted modified replacement PCB's to some models around the time this was built but I don't have access to any technical bulletins etc).

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The fridge fan should run all the time with exception of when in defrost mode and when the fridge door is open.

I would agree this suggests the PCB may be faulty if moisture is forming in the air duct and not being defrosted properly causing the flow restriction there by preventing the fridge from maintaining the correct temp.

I take it you have performed a fully 12 hour defrost of fridge and freezer, as I have often found this often resolves the issue you reported.

Customer: replied 5 months ago.
Hi, yes I've tried leaving it overnight once and then for a full 24 hours on another occasion (including removing rear internal panels to check for hidden ice and confirming that the drain pipe is clear). My main concern is that I don't want to change the PCB only to find there's a secondary sensor or similar at fault (though the PCB is fairly cheap and quick to fit so it's not the end of the world).

If your confident its running through defrost process properly, and the fan isnt running properly the only thing I can think of is the door sensor may be stopping the fan from running so worth double checking that, but if isnt showing signs of fault, I would change the PCB.

Customer: replied 5 months ago.
Thanks, I'm pretty sure the door switch is okay as the fan runs for a while when you manually press the button in (and the fridge light stays switched off), but since it's easy enough to access I will continuity test it first and (assuming the switch is okay) replace the PCB.

No problem, do let me know if you need anything else.

Customer: replied 5 months ago.
Hi, I've replaced the PCB (with a new genuine replacement direct from bosch) but the fan still only runs with the compressor or immediately after the door is closed.

Correct the fan should stop when you open the door, what is the current temp of the fridge?

Customer: replied 5 months ago.
I know the fan stops when you open the door, but when you close it again it runs for perhaps 30 seconds (just like it previously did) then stops, and I think it then only runs when the compressor does. I'd cleared the air duct of ice again when I fitted the PCB and the temperature dropped to about 8 degrees but it's sitting back around 13 now (the freezer is still maintaining approx. 1.5 degrees either side of whatever I set it to).

To confirm the duct had frosted up?

Customer: replied 5 months ago.
Sorry, I should have been clearer, I cleared the duct of ice BEFORE changing the PCB, with the new PCB fitted it got down to around 8 degrees but has now returned to 12-15 degrees, I've not had time to remove the back panel to inspect the duct since but I'm assuming it's iced up again as though the fan can be heard running the air flow from the outlet is very weak. I'm still concerned that the fan is not running all the time when the door is shut despite replacing the PCB - my suspicion is that the lack of airflow combined with a 20-30 degree temperature difference between the top of freezer/base of fridge is causing condensation to form in the duct.

The duct should also have an element to defrost ice, the fan would stop when the fridge is at the correct temp since its design to pull air through the duct and this wouldnt be constant, but wouldnt expect it to have stopped for more than 10 minutes at a time.

If the PCB has been replaced, I think the fault is more likely to be through the valve which allows air to circulate through the duct from the freezer compartment, or the defrost sensor as this would also stop the fan running while its running a defrost on the duct.

Stary by removing the fan checking the continuity going to it, then check the valve on the duct and then the continuity of the defost element and sensor.

Customer: replied 5 months ago.
Hold on a minute, my original question was whether the fan should run constantly, and your response was that it should do aside from when the door is open or it is on a defrost cycle, and it was on that basis (as the fan was not running constantly) that I replaced the PCB - if the fan should only run intermittently then I may have changed it unnecessarily as the new one doesn't seem to make the fan run any differently to the old one. I'm pretty sure there is no sensor for the fridge temperature anywhere, and the duct valve responds correctly, so I guess testing the defrost heater might be my next option, as far as I can see on the parts diagrams it's only the evaporator that has a trace heater and it doesn't extend into the duct.

The fan should run constantly your talking about a 5 min gap every hour, maybe 10 min if it coincides with the defrost cycle. From what you were talking about it was stopping for long periods of time which is not normal.

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Customer: replied 5 months ago.
It definitely has way more than 5 mins downtime each hour - I guess since i bought the part from Bosch they should be able to tell me if its working correctly/what it's programmed to do.