Why Your Microwave is Not Heating (How to Fix It)


fixing a microwave that is not heating

 

Why is My Microwave Not Heating?

A microwave may not be heating food due to a blown fuse or a faulty high voltage diode. Replacing these parts will often fix the problem:

  • Blown fuses
  • Faulty high voltage diode
  • Faulty door switch
  • Faulty magnetron
  • Faulty thermal Fuse
  • Faulty ceramic fuse

Before attempting to fix your microwave, confirm that there’s a real problem with it heating up.

  • Check the power settings. Be sure that the microwave isn’t on the low power setting, which could cause food to not be heated as strongly. Stop the microwave, then press the power button to ensure it’s performing at 100% power.
  • Check if it’s heating consistent. To ensure that the microwave is heating consistently, fill a microwave safe glass about half full and place it in the microwave on full power for 1 minute. Repeat this process several times with fresh water each time, and if the water is not consistently warm then there is a problem with the heating.
  • Check for power failure. If your home recently had a power outage, the microwave’s settings may be interrupted. If the microwave displays “PF” (power failure) or the numbers “8888” (code that indicates microwave lost power), simply press the CLEAR button then reset the microwave’s clock.

 

High Voltage Diode Problem

This is the most common reason a microwave stops working, especially if the microwave is making a buzzing or humming noise without heating.

The high voltage diode helps to power the magnetron by allowing current to only pass in one direction, giving the microwave the energy, it needs to work. It may give off a burnt smell if it’s shorted or open.

Fixing a microwave with this issue requires opening the device up, so it is recommended to call a licensed technician to test and repair it.

 

Door Switch Problem

The door to the microwave contains three or four door switches that activate when the door is closed and tells the microwave that it’s safe to begin heating. If the microwave has power but refuses to turn on, it’s likely that one of the door switches is not working.

To check if this the reason why the microwave isn’t heating, ensure that the light inside the microwave turns off when the door is closed. If the light doesn’t go off, call a licensed technician to replace the door switches.

 

Magnetron Problem

If the microwave heats at a low level or heats sporadically then stops, it could also be a problem with the magnetron. The magnetron is a key part of the microwave, as it produces the heat necessary to cook your food.

Problems with the magnetron could be caused by age, a faulty switch or a blown fuse. If the magnetron’s fuse has blown, there would be a chain effect of failing parts throughout the device.

Call a licensed technician to repair the magnetron and switch as it involves opening the microwave.

 

Thermal Fuse Problem

The thermal fuse cuts off power to the microwave when it overheats. Use a multimeter to determine if the thermal fuse has been blown and test it for continuity. If the fuse doesn’t have continuity, it has to be replaced. It cannot be reset and if it’s blown, it has to be replaced. It is recommended to call a licensed technician to replace the thermal fuse.

 

Microwave Repair Safety

Microwaves operate at a very high voltage, running up to 3400Vrms. This is 5 times more dangerous as the 1500V overhead line of a train because of the power proportional to the square of the voltage.

 

Individual safety precautions:

  • Wear electrical insulating rubber gloves and boots, although this doesn’t protect against 4000V.
  • Use the correct tools and ensure that they are in good condition.
  • Refrain from wearing loose clothing and jewelry, and tie back long hair.
  • Don’t do measurements at high voltage, as high voltage passes through the insulation of the probe easily.

Microwaves are high voltage appliances and should never be taken apart when plugged in. A microwave must always be earthed, so the metal chassis has to be connected with a separate cable with the earth pin in the wall socket.

Microwaves generate high voltages and dangerous amounts of radio frequency radiation. Attempting to repair a microwave yourself is not recommended. 

I recommend calling a licensed technician for any serious problems with a microwave, especially if it require disassembling the unit.

 

How to Fix a Microwave that Turns On but Stops Heating Evenly

 

High Voltage Diode

If your microwave turns on but doesn’t heat up it could be a problem with the high voltage diode.

How to test your microwave’s high voltage diode with a multimeter:

  1. Unplug your microwave before beginning inspection.
  2. Locate and remove your high voltage diode to test it for continuity. Accessing it requires removing the cabinet and discharging the high voltage capacitor.
  3. Set the multimeter to Rx1 and touch the probes to the terminals. After that, reverse the probes to check for continuity in the other direction. A reading should show continuity in one direction, but not the other.
  4. If the diode doesn’t show continuity or shows continuity in both directions, a replacement high voltage diode is necessary.

 

Door Switch

When the light on the microwave doesn’t go out after the door is closed, there may be a problem with the door switch. If this component doesn’t work, the fan and stirrer may still function, making it look like the microwave is still working.

How to test your microwave’s door switch with a multimeter:

  1. Disconnect the microwave from the power source.
  2. Verify that the door hooks trigger the door switch mechanically. If they do, remove the microwave’s cabinet, locate the door switch and remove it to test for continuity.
  3. Set the multimeter to Rx1, depress the actuator button and touch the probes to the switch’s terminals. This should produce a reading of 0, indicating continuity.
  4. If the test results do not match the above, replace the door switch.

 

Magnetron

A microwave that doesn’t heat up may have problems with its magnetron.

How to test the magnetron with a multimeter:

  1. Ensure the microwave is disconnected from its power source.
  2. Remove the microwave’s cabinet to access the magnetron. Discharge the high voltage capacitor, verify that the high voltage diode is working and the mounting bolts are tight. If no other issues are found, remove the magnetron to test if. (Do not attempt to take apart the magnetron, as it contains beryllium oxide in the ceramic insulators which can be fatal if it enters the lungs)
  3. With a multimeter set to Rx1, touch the probes to the terminals. This should show a reading of 2 to 3 ohms of resistance. Then, move one probe to touch the metal housing of the magnetron. This test shouldn’t indicate that continuity is present.
  4. If the results of test do not match the above, a replacement magnetron is required.

A microwave that doesn’t heat evenly could have problems with its components or it simply be due to food with different ingredients have different rates of absorbing heat. Also, microwaves absorb heat into the surface more easily than the center.

 

How to Fix a Microwave that is Not Turning On

Door Switch

A microwave that doesn’t turn on could have issues with the door switch. The door switch signals the other parts to turn on when the door closes and to stop running when the door opens.

How to check door switch:

  1. Unplug the microwave and remove the cabinet.
  2. The switches have wires attached to the terminals marked common (C) and normally open (NO). Check for continuity using a multimeter.
  3. Set the multimeter to Rx1, depress the actuator button and touch the probes to the switch’s terminals. This should produce a reading of 0, indicating continuity.
  4. If the test results do not match the above, replace the door switch.

 

Ceramic Fuse

If the microwave doesn’t turn on and has a blank display, there may be a problem with the ceramic fuse. The ceramic fuse is used to protect the components of the microwave in the event of a severe power fluctuation or critical fault with a component.

 

How to check ceramic fuse:

  1. Ensure the microwave is unplugged from a power source before inspection.
  2. The ceramic fuse is located inside the cabinet near the entry point of the power cord. Check for continuity with a multimeter.
  3. If the fuse has failed for no apparent reason, check major components such as the magnetron, high voltage diode, capacitor and transformer, associated wire terminals before changing the fuse.
  4. If the fuse failed when opening or closing the door, check for a shorted door switch, monitor switch or improper adjustment of the door latch assembly.
  5. If an issue is found, use only manufacturer suggested part numbers when replacing the fuse.

 

Thermal Fuse or Cut Out

Thermal fuses are to prevent the microwave from overheating. If your microwave doesn’t turn on, there could be a problem with the thermal fuse.

 

How to check thermal fuse:

  1. Disconnect the microwave from a power source.
  2. The thermal fuse is located inside of the cabinet, near the magnetron or oven cavity. Remove the cabinet and remove the wires from the terminals. Use a multimeter to check for continuity.
  3. If no continuity is found, replace the fuse.

 

Door Latch Assembly

A microwave that won’t turn on may have a problem with its door latch assembly.

 

How to check door latch assembly:

  1. Locate the door hooks that protrude from the door. The door hooks fit into the door latch holder by the frame of the microwave.
  2. If either of the hooks are broken or don’t have any spring tension, the switches won’t be engaged.
  3. The microwave won’t turn on if this happens and requires replacement of the assembly.

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