De Detrich Oven DOD789B4

Good Afternoon All

The above oven has a problem that I can not replicate or have seen:

Customer reports that the top oven is intermittent in operation and sometimes the (top oven) fan self-activates when both ovens are not being used.

Any ideas as to where the fault may be? e.g one of the power boards or the display board or both?

Having been to the customer a few times, I have never seen the fault occur.

Does anyone have a Service Manual for one of these that I could get access to?

Any help would be appreciated.

Thanks

Regards