I received an MK3S kit about 2 weeks ago. After about 2 weeks of solid printing, i'm now getting heater bed issues. The bed is heating and is getting power. However, the controller doesn't think the bed is getting hot and is triggering a panic. Googling around this seems to be a pretty common thermistor problem. The controller thinks that the bed is between 25 and 30 degrees no matter what.
Check that the tape on the bottom of the heat bed is secure and holding the thermistor to the bed. If the thermistor isn't firmly touching, you'll get this problem.
Okay I just took the heatbed off to check that. It is firmly affixed. I left it off and tried to preheat the bed to 60c and I found something very interesting. The portion of the heatbed that the termistor is underneath is actually cold -- it isn't heating up at all. While all other zones seem to be getting very hot (until the firmware shuts it off).
Could this actually be a headbed issue?
Wow - odd problem; great catch. Only one connection to the bed, so I'm confident the bed is internally defective.
Contact Prusa and ask for a new bed. Best way is to log in to the e-shop, there is a Chat window lower right on the page, be patient, explain what you've found. They'll ship you one for free, or might have something else you can check that I am missing.