The problem is no heating but the time is counting. Model MW840WF

If there is no heating please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is faulty, the temperature cut off is faulty or the main capacitor is short.The processor seems to work but there is no output trigger or switch on of the magnetron. can be failure of the relay or the magnetron. It is also quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Make sure that the program controller is working if not check power - step down supply, the pads are responding- if not faulty pads- or the door switches, if contact is not through the microwave will not work.

Now keeping clear of the microwave, switch on, use a cup of water inside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. replace with same type.
If there is heating without any issues, you can use the microwave safely

http://www.gallawa.com/microtech/mag_test.html