MAXTEMP Error [Simple Solution]
I didn't see this in a search, so I thought I would add it for reference.
Relatively new MK3S, suddenly got a MINTEMP error, and had a dead hot end thermistor (showing 0 degrees C and way of of spec on multimeter). Replaced it and all was good.
About 20 hours of printing later, and I started getting random temperature spikes while printing. They would usually go down, but it was starting to effect print quality, and I started not being able to get past two or three layers without a MAXTEMP error rearing its head.
Checked my connections, looked at wire routing, all was good.
I finally checked the grub screw that holds in the thermistor. There was NO resistance as I unscrewed it. I had only lightly screwed it in for fear of damaging the device, and hadn't done it tight enough. As the printer would move, the thermistor would get loose, under reporting the temp, and then after the temp would soar, it would re-connect and push it back down again.
By tightening the grub screw a little more than I had, it made better contact and solved the problem.
I can see this screw backing out on someone's machine over time, so if your temperature issue is intermittent, check the grub screw to make sure it is secure. It will save you downtime and the headache of re-running all those wires again.