Microwave faults: what you can fix, what you must not open

Where the line falls on a microwave

On most appliances on this site the line between what you can do and what needs an engineer runs somewhere inside the machine: a filter you can reach, a pump you cannot. On a microwave it runs at the outer cover, and the reason is specific. Behind that cover is a high-voltage capacitor that stays charged after the plug is out. Whirlpool's service documentation puts it in capitals — it is possible to come in contact with lethal high voltage when working with the transformer, capacitor and magnetron — and their current tech sheet says to be sure the capacitor is discharged before accessing any of those components. Samsung and Panasonic print the same instruction, and Samsung give a figure of around thirty seconds. Unplugging a microwave does not make the inside of it safe, which is not true of anything else here.

The second thing that makes this appliance different is the door. It is not a lid; it is a radiation shield, and the interlock chain behind it is a containment system that the product standard, BS EN IEC 60335-2-25, requires. A dryer with a cracked door still dries. A microwave with a damaged door, seal or glass is the fault, and the manufacturers' instruction is not to operate it.

Everything below sorts by those two facts.

What is yours

The socket and the plug fuse. Before blaming the microwave, test the socket with something else — a kettle, a lamp. No life there either means a breaker or RCD at the consumer unit, not the appliance. A UK moulded plug carries its own fuse, outside the machine, and changing it once is fair game. If the new one goes too, stop: that is information, not bad luck. The other fuse — the one on the board inside the cabinet — is a separate part behind the cover, and it is not yours on this appliance.

The door, from the outside. Work the door a few times and look at how the catch engages. If the door hangs low, moves in its frame, or only latches with a push, the switches behind the catch are very probably no longer operating in sequence. Whirlpool describe the mechanism: hinge screws work loose, the door sits out of position, and the pins on the door no longer reach the interlock switches when it is shut. Look along the gap between door and frame; wider at the top than the bottom means the hinge, not the catch. Check the seal and the sealing face in good light for splits, crushed sections, corrosion or heavy baked-on soiling, and clean them with a damp cloth and washing-up liquid — soiling is a named cause of leakage failing its test, and it is the one part of that you can put right. Nothing abrasive. If the door will not open, press the release squarely while pushing the door gently towards the frame rather than pulling; on a combination model that has just run a grill or oven function, wait, because some hold the door until the cavity cools. Do not lever it with a knife or screwdriver. Distorting the frame is a reason to stop using the microwave altogether, which is a far worse outcome than a broken catch.

Inside the cavity. The turntable, its coupler and roller ring lift out; a plate off its seat is the whole fault more often than a motor. Set into the wall or roof of the cavity is a small rectangular panel, usually a thin mica-coloured board. That is the waveguide cover, and it is the usual answer to sparking. Food splashes onto it, the deposits carbonise, carbon conducts, and the cover starts arcing against itself in the same spot every time, from a panel with visible brown or black scorching or a hole. Metal-caused arcing — foil, a fork, a gilt-rimmed plate, a twist tie — happens where the metal is and stops when it is removed; a failing cover comes back and gets worse. A cover that is only dirty often stops arcing once cleaned and fully dried. A cover that is scorched through or holed should be replaced before the microwave is used again: it is an inexpensive part, cut to size for the model, and on many machines it fits from inside the cavity without opening anything. Do not improvise with card or plastic, and do not scrub a mica board hard, because it delaminates.

The mug test. Put a good quantity of cold water in a microwave-safe mug, close the door, run one minute on full power, and feel the water. Sharp publish a fuller version for their machines — half a cup of water, one minute at P-100, then four questions: did the lamp come on, is the fan running, did it beep at the end, is the water hot. A no to any of those is a real fault. Hot water with a code on the display points at a protection rather than a breakage. Cold water with the lamp, fan and turntable all running is the classic microwave failure, and it is the point at which your part ends.

The plain instructions. Do not run it empty — the Whirlpool service manual prints "never operate the device without a microwave energy absorbing load" in capitals, and pressing start to use the timer with an empty cavity is how tubes get quietly weaker. Clear the vents; overheating protections on Sharp, Bosch, Miele and Whirlpool models name a blocked exhaust path or a housing without the specified ventilation before they name a part.

What is not

The internal fuse and the interlocks. Completely dead — dark display, no light, no fan — nearly always means the fuse on the board has gone, and what matters is why. The door operates three switches in a set order, and one of them, the monitor switch, exists purely to check that the main interlock did its job. When the door is opened and that main switch fails to respond, the monitor deliberately shorts the supply, and the fuse goes. Whirlpool's service documentation describes exactly this sequence. Read it for what it is: the appliance concluded it might not stop producing microwaves with the door open, and chose to kill its own supply. So the internet remedy — find the fuse, fit another — does something worse than fail. It puts power back into a machine whose door safety has just failed a test. Whirlpool's own instruction is to fix the mechanical cause and fit a complete new set of interlock switches, and their current tech sheet says the switches are not adjustable and all of them are replaced if any one has failed. A microwave that went dark the instant the door was slammed is the textbook case, and it is an engineer's job from that moment.

Everything on the high-voltage side. The light is on, the turntable turns, the food comes out cold. Whirlpool's service table lists it as "Oven seems working, food does not cook", and the causes are a magnetron with a broken or shorted filament, a broken transformer earth wire, bad contact at the magnetron terminals, or a high-voltage diode breaking down — which, Whirlpool note, gives a hum. A loud buzz appearing alongside the loss of heating points at the diode or capacitor; a silent failure is more typical of the tube. That is as far as diagnosis at home goes. The parts are behind the cover, and the cover is where the capacitor is.

Anything that needs the casing off. Sensors, control boards, inverter units, cooling fans, door motors on BSH's automatic-opening models, the damper on an over-the-hob unit. The code pages for LG, Samsung, Bosch, Whirlpool and Panasonic all end the owner's part at a power cycle for those, and Sharp's standing instruction, printed in every one of their manuals, is not to adjust, repair or modify the appliance yourself, because doing so may involve removing covers that provide protection against microwave energy.

Panasonic's lockout. On inverter models the appliance counts H97 and H98 events and, past a threshold, stops working for good until a service reset is done — even after the part has been replaced. The sequence is documented and needs no tools, and this site will not walk you through it: the counter exists to stop an oven being run repeatedly with a fault on its high-voltage side, and clearing it without fixing that is defeating a protection.

Is it leaking?

The worry that brings people to a microwave page deserves a straight answer. There is a limit, and it is in the standard: 5 mW/cm² at 50 mm from the surface, the figure BS EN IEC 60335-2-25 sets and the one Whirlpool Europe write into their own service procedure, which requires a machine over it to be withdrawn from use until it has been repaired and passed a safety test. Measuring it means a calibrated meter, a water load in the cavity, and a slow scan of the door, glass, seal and ventilation openings — a trade procedure, and a short one. Where machines fail is consistent: the seal and the surfaces it closes against, through wear, baked-on dirt, corrosion, arc damage, a twisted door or tired hinges.

There is no home version of that test. Ringing a phone inside a closed microwave tells you whether the box blocks a mobile signal — a different frequency at a minute fraction of the power — and proves nothing either way about the limit. Nor does a PAT sticker: leakage testing was dropped from the IET Code of Practice at its fourth edition as not being a matter of electrical safety, so if you want it done you ask for it by name. A dropped appliance, cracked door glass or a damaged seal means stop using it and have it tested, or on a cheap countertop machine, replace it. Whether it still cooks is not the question being asked.

Repair or replace

The code pages carry the brackets, and the pattern is consistent. An interlock switch set with a visit is typically £70 to £140. A magnetron is £40 to £90 as a part and £110 to £220 fitted; inverter units on Panasonic and Whirlpool models run to £250 or £300, most of the upper end being the labour to get a built-in out of its housing. A waveguide cover is £5 to £20 as a part, £70 to £120 with a visit. A latch assembly is £10 to £30, most of a £60 to £130 job being the visit. A leakage test as part of a visit is £60 to £90.

For a built-in or a good combination oven those figures are usually worth paying, because a replacement has to fit the existing housing. For a cheap countertop machine the visit alone may cost what a new one does, and buying a new one is the honest answer. What is never the answer, on any microwave, is a new fuse in a machine whose door switches have just failed, or carrying on with a damaged door because it still cooks.

Right now

Unplug it, and then leave it unplugged rather than trying anything else. If the door, the seal or the glass is damaged, or it has been dropped, stop using it. If there is fire or arcing inside, keep the door shut and pull the plug rather than opening it to look. The reset procedures each manufacturer does publish are in How to reset a microwave, and what the display means.

Sources

• Whirlpool Europe service manual for the AVM 512 microwave oven, document 4812 714 12457: the interlock and monitor switch system, "Oven seems working, food does not cook", the lethal-voltage warning, the load warning, the 5 mW/cm² leakage test and the conditions under which the oven must not be operated.
• Whirlpool tech sheet W11381054 Rev A (2020): discharging the capacitor, the pre-service safety checks (interlock operation, door closing, seal and sealing surfaces, hinges and latches, evidence of dropping), "Do not attempt to operate the oven if the door glass is broken", and the instruction to replace all interlock switches if any is defective.
• Samsung and Panasonic microwave service manuals on discharging the high-voltage capacitor, as quoted in this site's trade wording for the category.
• Sharp UK microwave manuals (the water self-check and the standing instruction not to remove covers).
• BS EN IEC 60335-2-25 (interlock requirement; leakage limit) and the IET Code of Practice for In-service Inspection and Testing of Electrical Equipment, 4th edition, on the removal of microwave leakage testing from its scope.
• Cost brackets are those on this site's cross-brand microwave pages (completely dead, not heating, sparking, door will not open, leaking) and the Panasonic H97 and Whirlpool F1E5 pages, each with its own sourcing.