This appears to be a common problem after frequent use.
The guide to diagnosis is normally you have used the oven for 24 to 48 months of operation, average of one to two hours a day.
If this sounds like it describes your environment, then;
1. turn off the power
2. remove the rear cover
3. examine the area behind the fan, the cables and connectors for obvious heat or electrical fault related damage.
4. should no obvious damage be visible, then you will require a replacement heating coil.
5. replace the covers, power the device
The description of the required part is 'element oven fan xxxx W'
(where xxxx is the wattage required)
You will need to contact your local Westinghouse spare parts provider and describe your fault, then request the part ''element oven fan ...' for your model.
Please note, the part required is not actually a fan, it is a double looped heating coil.
Should you have any doubts as to fitting it, then any qualified electrical trades person, or competent equivalent, should be able to assist.
Regards,
John Harris
August 2007