Repairing an electric kettle that doesn't always auto shut off

[Hogwild]

Hi all:

I would like to repair a friend's electric kettle which is showing 2 problems.

I think the first problem is the bimetallic disc that automatically turns off the kettle after the water has boiled.  It only seems to work when the kettle has a small amount of water in it. When you fill the kettle more, it doesn't shut off.

[inse]

What kind of kettle is it, the standard on/off type or an electronic one?
The kettles usually have two separate devices for turning off: the boiling switch is operated by steam and in addition an overtemperature switch which protects it in case of dry operation.
Mentioned here: https://en.wikipedia.org/wiki/Kettle#Electric_kettles
The German article has more details.
In case the bimetallic switch has failed, I think there‘s not much you can do…

[floobydust]

You can replace the temp switch, pics of it can help to source a new one, although just buying a new kettle would be less hassle.

Sometimes you get poor heat transfer to the temp switch - is it tight against the element base? No corrosion?
What I've also seen is a pressure port running from the lid down into the base switch. When the water boils (and the lid is closed) the steam pressure pushes that switch for auto shutoff. It's common for it to get plugged up with lime deposits. Look closely for a hole near the top and make sure it's not clogged.
A thermal fuse is there to cover when the kettle badly overheats.

[coppice]

A common problem is limescale build up in the tube that guides the steam to the sensor which detects the steam. You can get some quirky results with different water levels when that happens.