Microwave Runs but Won't Heat: Causes & What to Check

Quick checks before assuming a failed part

• Check the power level. A setting of 10 percent or a defrost mode pulses the energy on and off, and food barely warms. Reset to full power.
• Cancel any special mode. Some models have a demo mode, a sensor-cook program or a child lock that changes how the cycle runs. Your manual lists how to clear it.
• Close the door firmly and check that it latches. A door that doesn't fully engage can leave the interlock switches open, so the cycle starts on screen but the magnetron never turns on.
• Wipe the door seal and latch hooks. Food residue or a bent hook can keep a switch from closing.
• Try another outlet or reset the breaker. Rule out a tripped breaker or GFCI outlet.

The water test

Place a microwave-safe cup holding about 250 mL (1 cup) of room-temperature water in the centre of the turntable. Run it on full power for one minute. The water should be clearly warm or hot to the touch.

Never run the microwave empty. With nothing to absorb the energy, it can damage the magnetron. If the water stays cold, the heating circuit is not working. If you also hear a loud hum or buzz, or the lights dim briefly at the start, the high-voltage section is trying to run and something in it has failed.

What usually fails when a microwave won't heat

• High-voltage diode: works with the capacitor to convert the transformer's output into the high DC voltage the magnetron needs. An open or shorted diode gives a working display with no heat.
• High-voltage capacitor: works with the diode. A failed capacitor often shows as no heat, sometimes with a buzz or a blown fuse.
• High-voltage transformer: it may run hot, hum loudly or fail outright, which also means no heat.
• Magnetron: the tube that actually generates the microwave energy. A failed magnetron can give no heat, weak heat or a loud humming, and arcing inside the cavity can damage it.
• Thermal fuse or thermal cutoff: opens on overheating and cuts power to the heating circuit. On over-the-range units, blocked airflow or a failed cooling fan is a common reason it trips.
• Door switches and interlock assembly: a microwave has several switches, and a monitor fuse or switch that fails will stop the high-voltage circuit. These parts protect against microwave energy escaping, so they are never bypassed.
• Control board or relay: the board sends power to the high-voltage circuit. If the display and fan are fine but nothing reaches the transformer, the relay or board is suspect after the other parts are ruled out.

Sparks, burn marks and burning smells

Sparking inside is not the same fault as no heat, but the two can occur together. Burnt spots on the waveguide cover (the small panel on the cavity wall, usually on the right side) are a common source of arcing. Stop using the microwave if you see sparks or scorch marks, and have it inspected before it is used again.

A burning smell or smoke from an over-the-range unit also calls for switching it off at the breaker.

Safety: why the inside of a microwave is not a DIY job

The high-voltage capacitor in a microwave stores a lethal charge, and it can keep it long after the plug is pulled. Touching the capacitor, transformer or magnetron connections without discharging the circuit properly can cause a severe or fatal shock. Removing the outer cover is not a safe step for a homeowner.

Do not bypass a door switch or run the microwave with the cover off, and don't use it with a damaged door, seal or hinge. The checks above all happen outside the cabinet and with the unit unplugged where relevant.

What a technician checks

• Discharges the capacitor before touching anything inside the cabinet.
• Tests the thermal fuse, monitor fuse and each door switch for continuity.
• Tests the diode, capacitor and transformer windings against expected readings.
• Checks the magnetron for a shorted or open filament and for signs of arcing damage.
• Checks the control board output, then tests for microwave leakage around the door after any repair that involves the door or seal.

Repair or replace?

For an over-the-range or built-in microwave, repair is usually worth it. Replacing them is costly, and over-the-range units also have to be unmounted, refitted and re-vented. A failed fuse, diode, capacitor or door switch is a routine repair. For the general rules on when a repair stops making sense, see when to repair or replace an appliance and average appliance repair costs in Toronto and the GTA.

For a countertop microwave, compare the repair quote to the price of a comparable new unit. If the magnetron or transformer has failed on an older, basic model, replacement can be the better choice. The same applies if the cabinet is dented or the door is damaged. Ask for a written quote after diagnosis, before agreeing to any parts.

When to call a technician

We handle microwave repair for over-the-range, built-in and countertop units across Toronto and the GTA. If you want to know how a visit works first, read what to expect from an appliance repair visit.

• The water test fails after you have checked power level, door latch and special modes.
• The microwave hums, buzzes or trips the breaker when started.
• You see sparks, scorch marks, melted plastic or smell burning.
• The door doesn't close or latch properly, or the seal is damaged.
• It's an over-the-range or built-in unit that needs to be taken down from the wall or cabinet.