White countertop microwave on a kitchen counter

Generic photo: cottonbro studio · Pexels License

Microwave heats slowly or weakly

Confirm with a water test on High. Then rule out a low power setting and low line voltage, since a small voltage drop cuts output a lot. An aging magnetron is the most common part cause; poor connections and a failing thermal protector also show up.

Single source ~30 min advancedBrands: All

Before you start

DANGER: the HV capacitor can hold a lethal charge even when unplugged
Untrained users must not open the cabinet
Never measure the HV circuit while energized

Diagnostic steps

Unlock all 4 steps with expected values

Full library + AI assistant for $34.90, one time.

Get lifetime access

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

More microwaves guides

Component test · Microwave

Magnetron: filament and filament-to-case tests (microwave)

The magnetron turns high voltage into microwave energy. An ohmmeter can catch an open filament or a filament shorted to the case, but it cannot prove the tube makes full power. Rule out the HV diode, capacitor and transformer before ordering a magnetron.

6 steps ~40 minadvanced

Component test · Microwave

Microwave high-voltage diode test

The HV diode works with the HV capacitor to double the transformer voltage for the magnetron. It is made of many junctions in series, so an ordinary meter only shows it is not shorted. A shorted diode is a classic cause of no heat with a loud hum.

4 steps ~20 minadvanced

Component test · Microwave

Microwave HV capacitor and bleeder resistor

The HV capacitor has a built-in bleeder resistor meant to drain its charge, but bleeders fail, so always discharge by hand. A shorted capacitor blows the HV fuse immediately. Resistance tests catch shorts; a capacitance reading is better.

4 steps ~20 minadvanced