Bosch Microwave Not Heating? Here’s How to Fix It

Bosch microwave models 

This guide is applicable to most Bosch microwave models including:

CMG633BB1A
BEL554MS0A
BEL653MB3A
FEL053MS2A
CMG633BB1A
BEL554MS0A

Why is my Bosch microwave not heating? 

Microwaves are not much use once they’ve stopped heating. Sometimes a microwave will still run, but will not actually heat the food. There are a few things that may be causing this issue. If the power supplying the microwave is defective then the microwave will likely not turn on. This is a very common issue and usually comes with an easy fix. 

Other issues that may be resulting in your microwave being unable to heat include a broken door switch. If the door switch is broken your microwave will think the door is left open, and will not heat up. More serious reasons as to why your microwave is failing to heat include a broken high voltage diode, a broken magnetron, or a blown thermal cutoff. 

In this article we will cover how to check for each of these issues, and what to do to fix them yourself. 

What to do if your Bosch microwave has stopped heating 

1. Check the power source 

The easiest place to start when trying to diagnose the issue is to look at the power source. You can simply do this by plugging your microwave into a different power outlet. 

If your microwave begins to run as it should after plugging it into another socket, it is likely the problem was in the power source, rather than the microwave. An electrician or appliance professional can help you fix the power socket if this is the case. 

2. Check the microwave’s door switch 

A microwave door switch is designed to stop the microwave from running when the door is open. If your microwave is not running, or the microwave light does not come on when the door is open, it is a telltale sign that your microwave’s door switch is broken. 

Replacing a broken door switch is usually inexpensive. However, it is important that you check the user manual to purchase the correct part for your microwave’s model. 

If you suspect that your Bosch microwave has a broken door, switch follow these steps to replace it: 

1. Unplug your microwave from the power source 
2. Remove the control panel by unscrewing the face or rims of your microwave, be careful to keep track of what screws belong where
3. Remove the wiring that attach the control panel to the board
4. Unscrew the switch assembly located on the side, some will need to be slid out of place
5. Remove the connectors attached to the switch assembly
6. Test for the defective switch using an odometer, or isolate the broken switch by looking for any signs of damage including erosion 
7. Remove the faulty switch. This is usually done by unscrewing or unclipping it
8. Replace the defective switch with a new replacement switch 
9. Reassemble the microwave, plugging in all the necessary wiring and screwing in the brackets and face that was removed   
10. Test the microwave to see if it heats 

3. Check the high voltage diode 

The high voltage diode is a component of your microwave. This supplies voltage to the magnetron that is responsible for generating microwaves. The high voltage diode may need replacing if your microwave is running, but is not heating. Other signs include an electrical burning smell, or a buzzing sound that is coming from your microwave. 

If you suspect that there is a problem with your high voltage diode you can follow these steps to replace it:

1. Unplug the microwave from the power source 
2. Remove the control panel by unscrewing the face or rims of the microwave
3. Discharge the capacitor by touching the tip or a screwdriver, or pliers with an insulated handle to the metal
4. Carefully unscrew the diode from the frame and capacitor to remove it 
5. Install the new diode by using pliers to connect it to the terminal on the capacitor 
6. Screw the other side of the diode to the support bracket
7. Reassemble the microwave by securing all loose screws and inserting the panel back into place 
8. Check to see if the microwave is running and heating 

4. Check the magnetron 

As previously mentioned, the high voltage diode works alongside the magnetron to create microwaves which heat the food. If your microwave runs but does not heat, it may be that the magnetron is broken. This is usually a more serious issue that can be expensive to repair. However, if you have spare microwave parts, replacing the magnetron is a viable option. 

If you suspect your magnetron is broken you can replace it by following these steps:

1. Unplug the microwave 
2. Unscrew the microwave’s side panel. This will expose the magnetron, a small, metallic, box-shaped component that is screwed into the microwave
3. Discharge the capacitor by touching the tip of a large screwdriver, or pliers with an insulated handle to the metal
4. Carefully remove the wires attached to the magnetron.
5. Fit the new magnetron into the space, hooking it in and securing it with the same screws. Insure the magnetron is from the same Bosch microwave model
6. Plug the new magnetron in with the original wiring and reattach the side of the microwave 
7. Test the microwave to see if it heating

5. Check the thermal cutoffs 

The thermal cut off is a safety device that is built into your microwave and prevents the microwave from running when it has overheated. If the thermal cut off has tripped or is faulty, then your microwave will not heat. 

If you suspect your thermal cut off has been tripped you can fix it by following these steps:

1. Unplug the microwave from the power source 
2. Unscrew the microwave’s side panel and locate the thermal cutoff, you will likely find it on the magnetron
3. Use a multimeter to check the continuity of the thermal cut offs 
4. If the multimeter shows high resistance the the thermal cut off will need to be replaced 
5. Remove the faulty thermal cut off and replace it with a new one of the same make and model
6. Reassemble the panel and try running the microwave, to see if it heats