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JamesI, Handyman
Category: Appliance
Satisfied Customers: 2154
Experience:  I have repaired and maintained numerous items over the years, from fridges to freezers, radios to TV's.
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We have a weird problem with our Siemens built in oven.

Customer Question

Hi, We have a weird problem with our Siemens built in oven.
Assistant: Can you guesstimate how old your Siemens is? And just to clarify, what's the exact model?
Customer: Its a HB86P.70B, built in oven with microwave. Probably 7 years old +-1
Assistant: Do you plan on doing the work yourself?
Customer: If possible, I have had the cover off and replaced the fan a couple of years ago.
Assistant: Anything else we should know to help you best?
Customer: Yup, the symptom is that most of the time the thermostat works fine, and we do a lot of conventional cooking with it. When it has been at a temperature for a time (an hour?) and is turned down, it doesn't go down, if anything it gets hotter. Several burnt cakes...
Submitted: 11 months ago.
Category: Appliance
Expert:  JamesI replied 11 months ago.

The model number doesnt look right could it be hb86p570b

Customer: replied 11 months ago.
It could be, all I am reading off is the Instruction Manual, which may be generic. Its a combi conventional oven with built-in microwave. 240V. UK version
Expert:  JamesI replied 11 months ago.

This suggests most likely the thermostat (PTC sensor) is starting to fail.

A replacement can be found here for around £50. Its fitted to the rear of the oven.



Customer: replied 11 months ago.
Hi James
Sorry for the delay, we are on holiday. The first time the over-temperature happened we obviously thought it was finger-trouble, by the second time we weren't sure and now it is definitely when we turn the stat down after being at a high-ish temperature; it seems to lock on full. My thought is that its more a control problem than a thermostat probe one.
How confident are you that this will fix the problem? Specially as the stat is £44!
Expert:  JamesI replied 11 months ago.

I am going based on experience the majority 85% of the time it is the sensor.

If you measure the resistance of the PTC while the oven is running this should help show if the fault is with the thermostat.

Should the PTC seem to be working normally then it would suggest the fault is with the main PCB, most likely a faulty relay, this can also be checked by testing the element conductors and if heat is still e supplied when the themostat resistance is low.

A new PCB can be found here for around £200.

Expert:  JamesI replied 11 months ago.

So I can close the case please rate the service received, and get back in contact if I can be of any further assistance.