Symptoms
- Full power (100%) heats normally
- Lower power settings (like 30% or 50%, often used for defrost) heat inconsistently or not at all
- May be a newer style microwave marketed as having an inverter for even defrosting
- No unusual noise or sparking
Likely causes
- CommonFailing inverter boardThis specialized board controls the continuous lower-power output and is a common failure point in inverter models.
- PossibleSoftware or setting confusion with power levelsDouble-check you're actually selecting a lower power level and not just a shorter time at full power.
- PossibleMagnetron issue affecting all power levels, more noticeable at low settingsA weakening magnetron can sometimes show up first at lower power demands.
What to do
- Test full power heating first to confirm the microwave works normally at 100%.
- Deliberately select a lower power level (like 50%) for a longer time and observe whether heating is inconsistent, weak, or completely absent.
- Double check your manual to confirm you're using the power level function correctly, since some models require pressing power level before entering time.
- Note whether the defrost function specifically is affected, since that's the setting most likely to reveal this issue.
- If full power works fine but lower power settings clearly don't, this points to the inverter board specifically.
- Have a technician test the inverter board, since it's a distinct part from the magnetron on these models.
Tools and parts
Microwave inverter boardSearch: “microwave inverter board replacement”
When to call a pro
If full power works but lower power settings don't, have a technician test and likely replace the inverter board.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.