Bosch oven cooling fan comes on spontaneously; very hot resistor. NOW WITH PHOTO

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Hi all,

I have a Bosch oven with a cooling fan that will come on when it feels like sometimes for hours on end.

I've had the top off, and found a resistor that looks burnt. This same resistor is burning hot to the touch even with the oven sat idle. The resistor seems to be part of a thermostat which controls the cooling fan.

Is my problem the resistor, or is this just the symptom? Thanks guys.

(Ignore disconnected cable; that was me).

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Hi,

I may be completely wrong, but it appears to me as some sort of fault with the fan, or fan circuit, is causing a higher current draw than usual.

Is the fan stiff to turn? A good clean and some appropriate lubrication could free it?

If the fan was drawing a higher current, this would heat the resistor up, and that in turn would keep the thermostat active, running the fan for longer, further damaging the resistor!
A positive feedback loop! :)

Apart from the scorching on the resistor, the angle of that spade terminal doesn't look good, both issues suggest at the very least, the resistor will need changing.

...although you say the fan turns on randomly, is this at any time, not just after cooking?
That could suggest that the circuit is activating when it shouldn't.
A faulty, or stuck thermostat and a stalled fan, could allow current to pass continuously, until the resistor heated up enough to fully switch the thermostat and start the fan?

As I say this is just speculation from me and I could be wrong! :)
 
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Hi,

I may be completely wrong, but it appears to me as some sort of fault with the fan, or fan circuit, is causing a higher current draw than usual.

Is the fan stiff to turn? A good clean and some appropriate lubrication could free it?

If the fan was drawing a higher current, this would heat the resistor up, and that in turn would keep the thermostat active, running the fan for longer, further damaging the resistor!
A positive feedback loop! :)

Apart from the scorching on the resistor, the angle of that spade terminal doesn't look good, both issues suggest at the very least, the resistor will need changing.

...although you say the fan turns on randomly, is this at any time, not just after cooking?
That could suggest that the circuit is activating when it shouldn't.
A faulty, or stuck thermostat and a stalled fan, could allow current to pass continuously, until the resistor heated up enough to fully switch the thermostat and start the fan?

As I say this is just speculation from me and I could be wrong! :)
Thanks for detailed explanation; that makes sense.

Yes, fan will come on apparently spontaneously. The cooling fan comes on after using the oven as expected, but never switches off; I have to switch off the power to the oven. Then hours later I’ll switch power back on, and fan will start up after a few minutes even though oven hasn’t been used for many hours.

The resistor and thermostat can be bought together for £32. Is it worth a punt?
 
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The resistor and thermostat can be bought together for £32. Is it worth a punt?

It could very well be, depending on age and state of the oven etc.

If you have a multimeter, it may still be worth checking the current draw of the fan to see if it is still 'in-spec', and of course clean it / check it runs freely.

If the fan isn't a great expense, that may also be worth replacing?
Though I'm sure just replacing the 'stat unit will be fine!

Good luck :)
 
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