Bosch kettle wont turn off?

It may well be a factor - the missing filter.
Steam reaches the switch mechanism at the bottom down a pipe that descends from the top - if the filter is missing then more steam can escape through the open spout - as a test cover the open spout when it starts to boil with the base of a mug - see what happens - then you'll know...

My kettle has a missing filter and is reluctant to switch off.

The consumer magazine Which picks a £12 ASDA kettle as a Best Buy...

Good Luck...
February 2017