How it knows the water boiled (a pair of steam-powered ears)
An electric kettle has no thermometer — it has a pair of steam-powered ears: at a full boil, steam races up a dedicated channel and strikes a bimetallic strip, which bends with the heat and snaps the switch off with a "click". So the kettle's three classic ailments all live on this signal path: the plate wrapped in scale (heat can't escape — slow boiling; fake heat fools the sensor — trips before boiling); an unseated base / dry-boil protection tripped (no power reaches the plate at all); the steam channel blocked by scale flakes (the ears go deaf — the water boils and boils and never stops).
Match the evidence
- Boils noticeably slower, sounds muffled, a hard white ring on the plate → Scale on the plate: boil a 1:1 vinegar-water mix, let it sit 20 minutes, scrub gently with a soft brush, rinse — then boil one full kettle of plain water and dump it, or your next cup tastes of vinegar. Monthly in hard-water areas.
- Trips before the water boils, or no lights at all → First wipe the kettle's underside and the base contacts dry (moisture = bad contact) and reseat it until it "clicks"; a kettle that once boiled dry is in dry-boil protection — it resets itself once fully cooled. Add water above the minimum line before the next boil.
- Boiling for ages and never clicks off → The steam channel is blocked by scale flakes, or the lid isn't seated (steam leaks out, the ears get no signal): seat the lid firmly, clear the steam port with a thin probe, and flush the channel with vinegar. This direction is dangerous — a kettle that never stops is heading for a dry boil. Deal with it soon.
One non-negotiable safety line: a kettle is an appliance where water and electricity share one body — if the base of the kettle weeps water or the body is cracked, stop using it immediately. No tape, no sealant — replace it. Frequent tripping of your home breaker or a hot plug sits on the same line. Keep your own work strictly to "water and power fully separated": descaling, wiping contacts, cooldown reset. Anything sealed inside the base is not DIY — and for budget kettles the repair quote usually beats the price of a new one ($25–60), so even the tech will tell you to replace it.