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LG Microwave Not Heating? Here’s Why (+ How To Fix)

LG microwaves have an excellent reliability rating based on a Consumer Reports survey that ran from 2011 to 2021. But what could cause these highly reliable appliances not to heat?  

These are the 10 most common causes of an LG microwave not heating: 

  • Power supply issues
  • An active ‘Child Lock’ feature
  • The ‘Demo Mode’ is activated
  • Door or functionality errors
  • An active timer instead of a cooking function
  • Door switch failure
  • A burned microwave diode
  • A burned-out magnetron
  • A blown thermal fuse
  • A burned-out high voltage capacitor

Some of these issues can be quickly fixed. Others are quite complex and need to be resolved by a certified technician. In this article, you’ll learn more about which issues you can fix yourself when your LG microwave is not heating and which ones should be left to the professionals.

Power Supply Issues

Your LG microwave needs consistent and sufficient power to work properly. If the appliance has any power supply issues, the microwave won’t heat.

Power issues that lead to your LG microwave not heating include:

  • A loose power cord
  • A faulty wall outlet
  • A tripped circuit breaker
  • An improper wall outlet

How To Fix LG Microwave Power Issues

The best way to troubleshoot LG microwave power issues depends on the specific cause of the problem.

You can do one or more of the following fixes:

  1. Check if the microwave power cord is securely plugged into the wall outlet and isn’t loose. If the power cord is loose or unplugged, ensure that it’s plugged in all the way.
  2. If the microwave power cable is well-plugged into the wall outlet, check whether the outlet is receiving power. You can plug in a different appliance to see if it powers on. If not, call a certified electrician to assess the outlet. If the outlet has power, you can check for other possible power issues.
  3. Open the circuit breaker housing and check if the microwave switch is tripped (i.e., turned downwards instead of upwards or to the side and not aligned with other buttons). If so, turn the switch back on. Don’t force the switch back on if it’s hard to push or trips again immediately. Instead, call in a trained electrician to check the problem. 
  4. If the switch doesn’t seem tripped, turn it off and then back on to reset the circuit breaker.
  5. If your microwave is connected to a fused electrical supply, check the fuse for signs of damage (e.g., dirty fuse glass or damaged filament) and replace the fuse if needed.
  6. If you’re just starting to use the microwave but it won’t heat, ensure that you have a proper wall outlet for the microwave’s power requirement. 

An LG microwave should be plugged into a dedicated AC outlet with 120V, 60Hz supplied by a 15-20 circuit breaker or fused electrical supply. If you’re not sure whether the outlet you’re using meets these requirements, call a certified electrician and have a proper outlet installed.

Alternatively, you can request an LG repair service for your microwave if none of these troubleshooting procedures resolves your appliance’s heating problem.

The ‘Child Lock’ Feature Is Active

If your LG microwave is receiving power but won’t start heating when you hit the ‘Start’ pad, the ‘Child Lock’ feature might also be activated.

Child Lock is a safety feature designed to prevent children from playing with or locking the control keys when you clean the appliance. You need to deactivate the feature to get your microwave heating again.

How To Deactivate LG Microwave ‘Child Lock’ Feature

When the ‘Child Lock’ feature is active, you’ll see ‘L’, ‘Loc’, or a lock icon on display depending on your model. To deactivate the feature, tap the START/ENTER pad and hold it for 4 seconds until the lock icon/’L’/’Loc’ disappears. Your microwave should then heat when you make the required settings and hit the Start button again.

If pressing the Start button still doesn’t heat the microwave, reset the appliance by turning it off and unplugging it for 3 minutes. Then, turn it on again to test if it heats up this time.

The ‘Demo Mode’ Is Active

If your LG microwave is responding to the Start button, but the appliance doesn’t heat when you test the microwave even though the timer is counting down at an accelerated rate, it’s likely that the microwave’s ‘Demo Mode’ feature is activated. This is especially common when you’ve just purchased the appliance and are using it for the first time.

Therefore, you must deactivate ‘Demo Mode’ to get the microwave to heat.

How To Deactivate LG Microwave ‘Demo Mode’

To deactivate LG microwave ‘Demo Mode’, turn off the appliance and unplug. Leave the microwave unplugged for 10 seconds, then plug and turn it back on. Test the appliance by heating a cup of water (using a microwave-safe container) to see if it starts heating again.

Door or Functionality Errors

Your LG microwave won’t heat if it has a ‘DOOR’ or a functionality error (i.e., F and a number). Below are the common errors you’ll see on your LG microwave display when it’s not heating and what they mean.

Error CodeMeaning
DoorThe microwave door is open
F-1The thermistor detects a fault in the convention function. 
F-2The thermistor has a short issue, or the microwave isn’t heating 10 minutes into the cooking cycle
F-3The thermistor has a short issue, or the microwave temperature doesn’t go down after a completed cooking cycle.
F-4Humidity sensor issue
F-5Damper switch sensing fault in the convention function

To troubleshoot these errors, you’ll need to reset your LG microwave.

How To Reset Your LG Microwave

If you have a ‘DOOR’ error, all you have to do is close the microwave door to clear the error.

If you have any of the ‘F’ errors I mentioned above, follow the steps below to troubleshoot the oven and run the water test I mentioned earlier (i.e., heating a cup of water in a microwave-safe container):

  1. Cut the power supply to the microwave by turning off the switch and unplugging. Alternatively, you can turn off the circuit breaker.
  2. Leave the microwave off or unplugged for 15 seconds.
  3. Plug the microwave back into the outlet and turn it on.
  4. Fill an 8-oz (236.58 ml) microwave-safe cup with water and put it into the microwave.
  5. Let a cook time setting run for 30 minutes. 
  6. Check if the water is heated and if the error code reappears when the set time is complete. If the water isn’t heated or the error reappears, your microwave needs professional repair.

Active Timer Instead of Cooking Function

When setting a cooking function on your LG microwave, it’s possible to accidentally activate the timer instead of the cooking function. When this happens, the microwave won’t heat, but the timer will keep counting down.

How To Fix Timer Instead of Cooking Function

You can tell if the timer was set instead of a cooking function if the timer is counting down but the microwave isn’t heating. If you notice this, just press the TIMER ON/OFF button. This will cancel the timer function, and you can then set the desired cooking function. 

Door Switch Failure

Most microwaves come with multiple door switches. When you close your microwave door, the switches activate in a certain sequence to make sure the door is properly closed. In case any of the door switches fail, the microwave won’t heat until the faulty switch is replaced. 

You can tell that a microwave door switch has failed if the control panel works but the appliance won’t heat. You can also test the switches for continuity before you decide to replace any of them.

How To Test and Replace an LG Microwave Door Switch

Caution! Microwaves are high voltage appliances and can retain power in the capacitor even after the appliance is turned off. Therefore, I recommend that the procedure for testing and replacing an LG microwave door switch (and other internal parts, if needed) be left to a licensed technician.

But if you want to test your LG microwave door switch yourself, here’s what you can do: 

  1. Turn off power to the microwave at the circuit breaker. Wait for a few minutes to make sure no current is running into the appliance.
  2. Open the microwave door.
  3. Loosen the screws securing the microwave grill and slide it to the left to remove.
  4. Unfasten the screws on the left and at the rear of the control panel.
  5. Lift and pull the panel forward to release. 
  6. Unscrew the switch assembly and press the tab and actuator to isolate the switch.
  7. Remove the connecting wire to take out the old switch. 
  8. Rotate the multimeter dial to the lowest reading for ohms of resistance.
  9. Touch the common terminal (the one with the letter ‘C’) with one probe and the normally open terminal (the one with the letter ‘O’) with the other probe. A zero ohms reading means the switch has continuity. 

If the multimeter display doesn’t show any significant change or the needle on an analog multimeter doesn’t show any reaction, the switch has failed. Repeat the test with the other switch(es) and replace any faulty switch.

To replace your faulty LG microwave door switch:

  1. Attach the connection wire to the new switch and snap it into the assembly.
  2. Readjust the switch assembly and tighten the holding screws.
  3. Reattach the control panel and fasten the rear and the front-left screws.
  4. Slide the grill to the right to position it in place and tighten the screws.
  5. Close the microwave door and reconnect it to the power plug. Your microwave should resume heating.

You can also watch these steps in action via this YouTube video from Repair Clinic:

A Burned Microwave Diode

Microwaves have a diode that converts the AC to DC, increasing the voltage required to power the magnetron that heats the food. If this diode burns, the magnetron won’t receive enough power, and the microwave won’t start heating. 

You can tell if a microwave diode is burned if it has signs of damage like a crack, blistered spot, or the diode is split in half. If the diode has no signs of damage, but you suspect it’s the cause of your LG microwave not heating, you can also test the diode for continuity.

How To Test and Replace a Burned LG Microwave Diode

Safety Precaution: As with other LG microwave repairs, it’s best to leave the testing and replacing of a burned diode to a certified technician. 

To test a faulty microwave diode:

  1. Turn off the power to the microwave.
  2. Open the microwave door.
  3. Take out the glass turntable and support.
  4. Place the microwave on a table with the rear panel down. Spreading a towel on the table will protect the appliance. 
  5. Remove the screws on the bottom panel, detach the LED light wire from the panel and place the panel aside.
  6. Turn the microwave upright and open the door.
  7. Remove the screws holding the front vent and set it aside.
  8. Remove the screws of the cabinet cover and the blower assembly cover and slide the latter to the back so you can take out the cabinet cover.
  9. Place the ends of a pair of pliers on the capacitor terminals to release any electrical charge that could still be in the component. Make sure you’re holding the insulated part of the pliers when doing this to prevent electrocution. 
  10. Remove the screws securing the diode’s grounding wire so you can detach the old diode.
  11. Use a multimeter and a 9-volt battery to test the diode for continuity. Hold the black probe on one end of the diode and let the other end of the diode touch the battery’s negative terminal. Touch the battery’s positive terminal with the red probe.

If the multimeter doesn’t show signs of continuity, reverse the test so that the black probe now touches the opposite end of the diode and the other end touches the battery’s negative terminal. 

Remember that the diode only passes current in one direction. So, if no continuity is recorded from both sides, the diode is faulty. At this point, you can replace the diode.

To replace a faulty microwave diode:

  1. Attach the new diode to the capacitor terminal and screw back the diode grounding wire.
  2. Place the cabinet cover and screw it on. 
  3. Slide the blower assembly back in position and fasten the screws. 
  4. Place back the front vent and tighten the screws.
  5. Turn the microwave, so it lies on the rear panel, then reconnect the LED light wire to the bottom panel. Fit the panel back into place and tighten the screws.
  6. Put the microwave in its place and turn on the power. Test to check if it resumes heating.

You can have a better idea of how to replace a microwave diode in this Repair Clinic video:

A Burned-Out Magnetron

Your LG microwave magnetron is the component that works with the diode and the high voltage capacitor to generate the microwave frequencies that heat or cook the food. Due to high voltage, the magnetron can burn out. In that case, you’ll have to replace a burned-out magnetron.

How To Replace a Burned LG Microwave Magnetron

Again, it’s better to let a licensed technician replace a bad magnetron to prevent electrocution. But if you want to replace a microwave magnetron on your own, follow the steps below:

  1. Turn off and unplug the microwave.
  2. Place the microwave on a sturdy working surface. Spread a towel to protect the appliance.
  3. Remove the screws on the microwave grill.
  4. Open the microwave door, pull the grill left, and lift to remove it.
  5. Pull out the turntable and support.
  6. Turn the microwave so it sits on the back panel, and unscrew the bottom panel.
  7. Detach the connection wires from the bottom panel and set it aside.
  8. Set the cabinet aside. Put the microwave upright and unthread the cabinet screws.
  9. Touch the terminals or the capacitor with the ends of a pair of pliers to release any stored voltage in the capacitor. Make sure your hands are on the insulated part of the pliers.
  10. Detach the magnetron connection wire and remove the mounting screws.
  11. Pull the magnetron from the bracket and detach the wires on the thermo protector.
  12. Unscrew the thermo protector bracket from the magnetron.
  13. Place the thermo protector and the bracket on the new magnetron and fasten the screws. Check that the seal at the antenna base is in its place, and reattach the wires to the thermo protector.
  14. Mount the new magnetron on the bracket and screw it on. Reattach the connecting wire.
  15. Replace all the other parts starting with the cabinet, the connection wires on the bottom panel, the bottom panel, the grill, the turntable, and support (in that order).
  16. Place the microwave back in its usual place and reinstall power. Perform the water test to check if it heats.

This Repair Clinic video should give you a clearer idea of how to perform the steps above.

A Blown Thermal Fuse

The thermal fuse in your LG microwave turns off the power when the appliance overheats or has a sudden power surge. If the fuse is blown, your microwave won’t run. 

You cannot reset a blown thermal fuse. You need to replace it.

Note: Replacing a thermal fuse may look easy. However, because microwaves can retain high electricity voltage in the capacitor, it’s best that thermal fuse replacement is left to a trained technician. 

How To Replace an LG Microwave Thermal Fuse

To replace an LG microwave thermal fuse:

  1. Switch off the power to the microwave at the circuit breaker and unplug the power cord.
  2. Loosen the screws holding the front vent and set it aside. 
  3. Unscrew the grill that protects the fuse area and slide left to remove it. You can now access the thermal fuse.
  4. Carefully pull out the fuse, which is a small cylindrical device with metal terminals at each end.
  5. Test the thermal fuse for electric flow using a multimeter. Place each probe of the meter on a terminal of the fuse at each end. The meter should be set to the lowest ohms setting. If the meter doesn’t show any change, the fuse is dead and should be replaced. 
  6. Take the new fuse and fit it into where the old one was, making sure it clicks into place. 
  7. Replace the grill and the front vent.
  8. Power the microwave and test to see if it resumes heating. If your microwave fuse keeps blowing, get the high voltage capacitor checked for any malfunctions.

A Burned-Out High Voltage Capacitor

The high voltage microwave capacitor works with the diode to convert the AC voltage to DC and increase the voltage of the magnetron to heat the food. If the capacitor is burned out, the high voltage circuit won’t work, and the microwave won’t heat. 

A burned high voltage capacitor will show signs of damage, such as an oily film on the capacitor surface or a burned appearance. These are obvious signs the capacitor needs to be replaced.

How To Replace an LG Microwave High Voltage Capacitor

Caution! The high voltage microwave capacitor must be tested and replaced by a certified technician. 

To replace a high voltage microwave capacitor:

  1. Turn off and unplug the microwave.
  2. Place the microwave on a table to facilitate the work.
  3. Open the oven door.
  4. Take out the glass turntable and support.
  5. Remove the screws on the microwave grill and slide it left to take it out.
  6. Turn the microwave so it lies on the rear panel.
  7. Unscrew the bottom panel and detach any wires connected to the panel. Set the panel aside.
  8. Turn the microwave, so it sits upright on the table. 
  9. Unscrew the cabinet.
  10. Release any electrical charge that could still be in the capacitor by placing the ends of a pair of pliers on the capacitor terminals at the same time. Be sure not to touch the pliers’ metal part to prevent electrocution. 
  11. Mark the position of the capacitor connection wires and detach them. 
  12. Remove the screws securing the capacitor’s mounting bracket and take out the capacitor and bracket.
  13. Transfer the bracket to the new capacitor and screw it in the place of the old one.
  14. Reconnect the wires and reassemble the cabinet and grill.
  15. Place back the glass turntable and support and close the door.
  16. Set the microwave in its usual place and reconnect the power cable. Turn on the power and do the water test to see if the microwave starts to heat. 

Watch this Repair Clinic video from YouTube to get a better idea of how to perform the above steps:

Final Precaution: As I’ve said repeatedly throughout the article, replacing LG microwave internal parts can be dangerous because of the appliance’s high voltage. It’s best to hire a certified technician to do microwave parts testing and repair, even if you think you can do some of the steps I outlined in the article on your own.

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  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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