This is crazy interesting but I didn't quite understand, when the timer runs out the head doesn't work anymore or you just get an LED notification to get a new one, to me at least there's a big difference in the two!
Depending on the model, it usually blinks an LED and vibrates a pattern to let you know it's time for replacement. It's definitely not a hard lockout or anything.
The brush heads for advanced models also tell the brush which cycle to use and which intensity to vibrate at (e.g., for tongue cleaning brushes vs normal brushes).
