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This indicates an issue with the Magnetron that is faulty and will need replacement.
When you turn on the Microwave it starts and then after few seconds magnetron kicks in however, if control detects magnetron has not started to heat it will turn itself off or even trip the circuit.
When magnetron is faulty the Invert board is replaced as well.
The parts are available for £280 + repair charges.
Looking at the age of this appliance and cost of repair, it is better to buy new one then to spend this amount on repairs.
Sorry about the bad news.
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