Magnetron fan runs all the time

The issue you're experiencing with your Bosch convection microwave not heating in convection mode could be caused by a few different things. Here are a few things to check:

1. Check the settings: Make sure that the microwave is set to convection mode and that the temperature and time settings are correct.
2. Check the element: The bottom heating element is responsible for providing heat during convection mode. Make sure that the element is not damaged or burnt out. You can visually inspect it, if it is damaged it will need to be replaced.
3. Check the fan: The fan is responsible for circulating hot air during convection mode. Make sure that the fan is not blocked or damaged. If it is, it will need to be replaced.
4. Check the control board: The control board is responsible for controlling the functions of the microwave. A malfunctioning control board could prevent the microwave from heating in convection mode.
5. Check for a blown thermal fuse: A thermal fuse is a safety device that cuts off power to the oven if it overheats. If the thermal fuse is blown, it will need to be replaced.
6. Check for a blown thermal cut-off: A thermal cut-off is a device that cuts off power to the oven if it overheats. If the thermal cut-off is blown, it will need to be replaced.

If your microwave is a 220 then reset the breaker by turning it off for 10 seconds and then back on. Then check it. Since you replaced the magnetron, the signal to the mag is interrupted. Did you replace the control panel? If so, then look to see if it is wired back in order. Does the fan operate when you open and close door? If not, then check door switch.

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 the switch contact are not good then it will not trigger the processor to switch on the waveguides. If the lights or fan work without any display or action of program being triggered to start then it can be a possible fault on the main board or the processor. Sometimes insect secretion can short the board and display panel and so the processor can work in such an erratic manner. If so clean , solder and reset the board.

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. 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. Check the wave guide covers, if food had deposited
then it must be checked or replaced.

See link: http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html

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

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. 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. Check the wave guide covers, if food had deposited
then it must be checked or replaced.

See link: http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html

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

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. 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

Sorry to read about your problem, I hope this helps you out.
you could have one of the following issues

1. Blown fuse in HV transformer primary circuit or HV fuse (if used).
2. Bad connections (particularly to magnetron filament).
3. Open thermal protector or thermal fuse.
4. Open HV capacitor, HV diode, HV transformer, or magnetron filament.
5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
6. Damaged protective VDR from filament to chassis (not commonly used).
7. Defective HV relay (not commonly used).

Microwaves must be serviced by technicians due to the inherent dangers involved in repairing these appliances. Internal capacitors can retain a lethal electrical charge even though the unit is completely unplugged. A microwave radiation leakage test must be performed on the unit following any internal component repair.

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Hi,Everything operates normally, but the oven shuts off after varying amounts of time. This could be a faulty magnetron, bad cooling fan (or just built up dust and grime block ventilation grilles), bad thermal protector, faulty controller, some other intermittent component, or bad connections.

• If resetting it allows cooking to resume immediately, if even for a few seconds, I would not suspect the magnetron or thermal problem as no cool down time is required. It could be bad connections in the controller or elsewhere, a marginal door interlock switch, or a controller problem. Jiggle the door to see if this will cause it to shut off.
  
• If the magnetron was overheating, you would not be able to resume cooking until it cooled and the thermal protector reset. If it just stopped working (i.e., the filament opened), everything would appear normal but there would be no heating. If the magnetron were shorting, there would likely be a loud hum associated with the periods where there was no heat.
  
• If it is not possible to resume cooking for a few minutes indicating that something needs time to cool off, then the magnetron could be faulty but check for the obvious cooling problems first: blocked or dirty ventilation grill. Determine if the magnetron cooling fan is operating by listening for its sound or looking through the ventilation opening in the back of the oven. If it is not, there could be a broken or weak belt, gummed up or lack of lubrication, other mechanical problems, a bad motor, or bad connections.
  
• Extremely high power line voltage may also result in overheating on a poorly designed or oven where the components are marginal.
  
• Make sure the stirrer fan is turning normally. Should it gets stuck, some models may sense this and shut down/restart.

Hope it helped..
Thanks for using fixya....

If there is no action 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. 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

Hello, If everything comes on when it is started, like the light in the cavity and the fan (which one can hear running), and the timer counts down but it doesn't heat, the problem is probably isolated to the high voltage section of the oven.the magnetron i guess is kind of faulty and this is the most expensive part in a microwave oven.
If the display is on and the oven won't come on and run, there may be a problem in the door interlock system or the thermal protection system.
wish yo good luck,

take care.