Why Is My Microwave Sparking Inside?

Short answer: stop using the oven. Sparking inside a microwave is almost always metal in the cavity, burnt food residue, or a damaged waveguide cover. The first two are easy to rule out yourself. The third needs replacing, and running the oven with a burnt cover can destroy the magnetron.

Check these three things first

1. Metal in the oven

The obvious culprits are foil, a fork left in a bowl, or a twist-tie. Less obvious ones catch people out regularly: crockery with a metallic gold or silver rim, some takeaway containers, and mugs with metallic glaze.

2. Baked-on food residue

Splatter that has carbonised on the cavity wall or roof will arc. Carbon conducts, which is the whole problem. Clean the interior thoroughly and try again with a cup of water inside.

3. The waveguide cover

This is the small rectangular panel — usually mica, a thin beige or grey board — on the inside wall or roof of the cavity. It shields the opening where microwave energy enters. Over years it absorbs grease and steam, and eventually it chars.

Once burnt, it arcs. If the panel looks blackened, scorched, blistered or has a hole in it, that is your answer.

Why a burnt cover matters more than it looks

The waveguide cover is one of the cheapest parts in the entire oven. But if you keep using the microwave while it is arcing, the arcing works its way back toward the waveguide opening and the magnetron behind it.

That turns an inexpensive part into an expensive one. This is the clearest example in appliance repair of a small fault becoming a large bill purely through delay.

Less common causes

• A damaged or rusted cavity. Where the paint has chipped and the metal beneath has rusted, arcing can start at that point. If the cavity has rusted right through, the oven should be replaced rather than repaired.
• The rack or shelf touching the wall. A metal grill rack that has shifted so it contacts the cavity side will arc.
• A failing stirrer or antenna assembly above the waveguide cover.
• A damaged door seal or hinge, which can also cause arcing near the door frame. This one is worth taking seriously, because the seal is what keeps energy inside the box.

Can you replace the waveguide cover yourself?

Physically, it is often just a couple of screws or clips. The reason to be careful is what sits behind it. You are working immediately next to the waveguide opening and, depending on the model, close to the high-voltage components — and a microwave capacitor can hold a lethal charge after unplugging.

If the part is genuinely accessible without removing the outer casing, some people do it themselves. If it means taking the cabinet off, do not.

What not to do

• Do not cover the burnt panel with foil or tape. Both make it worse.
• Do not keep using the oven “carefully” until it is convenient to fix. Each arcing cycle does more damage.
• Do not run the oven empty to test whether the sparking has stopped. Put a cup of water in.

Preventing it

Almost every sparking fault we attend traces back to the same thing: uncovered food. Cover dishes, wipe the cavity while it is still warm, and glance at the waveguide cover occasionally when you clean. A cover that is discoloured but intact is fine; one that is blistered or holed should be replaced before it takes the magnetron with it.

Get the sparking fixed

Call +91 82740 66348 for doorstep microwave repair across Kolkata. Tell us the brand and model number and we will bring the right waveguide cover with us where we can.

Frequently asked questions