I own a Prusa MK4S and it's been perfectly fine for a year or more, but recently I checked on an in-progress print and found that it had detached from the sheet. Ever since then, it hasn't got up to temperature.

If I start a PLA print then the bed does heat up, but only to somewhere around 40° (I don't have a way of actually measuring it so that's just an estimate: it's warm but not hot to the touch). At the same time, the printer display claims that it's sitting at 60°.

The heatbed itself seems to be working fine: if I go into Control and increase it past 60° then it does heat up, but it seems to report itself around 20° hotter than it actually is.

The self-tests aren't reporting any issues, and I've tried a factory reset and full recalibration to no avail. The thermistor is still firmly attached to the bottom of the bed.

I suspect this could be a thermistor failure but I'm hoping for a second opinion before I buy a new one. Has anyone else run into this sort of thing?