Usually when a microwave is on and runs but does not heat there is a substantial humming associated with the symptom. If you do not have that tell tale hum you have a control problem with the relay that provides power to the magnetron.

It is most likely the Smart board Item 12
http://www.appliancepartspros.com/partsearch/model.aspx?model_id=5227571&diagram_id=22974425#d22974425

BUT the Capacitor, the diode, the 3 thermostats and the magnetron all need to be ruled out as possible failures before changing the Smart Board.

See link below:

Cap = Item 49
Doide = Item 50
3 Thermostats = Items 110 (2ea) and 115
Magnetron = Item 73
http://www.appliancepartspros.com/partsearch/model.aspx?model_id=5227571&diagram_id=22974428#d22974428

It will take a meter and some resistance readings to isolate the thermostats, diode and capacitor.

The thermostats should read continuity

The diode should read continuity in one direction

The capacitor must be shorted several times to avoid ruining your day and possible injury.
The capacitor should make an ANALOG meter jump each time the leads are reversed.

This should give you an idea what to expect for repairs.

Thanks for using FixYa!

Kelly

In all likelihood the mangnetron has failed, or perhaps even the Power Supply. or a fault that is shutting the system down for safety reasons You need to have it serviced by a reputable service centre/man. Ensure you get a firm "Quote" before proceeding, and remember these days the cost of repair can equal or exceed the price of a new one.