Usually if a microwave isnt heating it is the magnetron. It can be tested with a multimeter. Set the meter to ohms 1k test between the two termnials.(uaually a PINK and a RED wire) meter should go to zero. Test between each termnial and the magnetron housing. meter should show open circut. if it does magnetron is good. There is also a diode that comes off from the capacitor and sometimes burns out. No real way to test the diode. It costs About $8.50 while the magnetron cost about $54.00. There are also some thermostat switches in there that can be tested with a multimeter(ohms 1k) post to post the meter should read 0 resistance. These switches run about $17.00. Be careful working around the capacitor and other electrical compoents as they often hold electrical charges even while unpluged. To purge them from electrical charges use a insulated screw driver and arc it betrween two posts. It will "POP" purging the electrical charge. If you need part # I have them just ask. Best of luck:) !