The Original Prusa XL will display the message "Extruder Maxtemp error: Check the thermistor wiring for potential damage" if the printer's nozzle temperature goes above 295 ºC at any point. This is a safety feature to prevent printer damage and fire. In this case, the printer will stop the heating and display the message on the screen. The printer then waits for the user to inspect all parts and if needed fix the issue.
Error name: Hotend Maxtemp error
Error code: #17206
The error might indicate some issue with the wiring, either the heater or the thermistor cables are possibly damaged.
Before breaking out the multimeter you should check the Dwarf board, where the thermistor and heater reside, to ensure that no wires have broken off, or are disconnected.
Place an empty Prusament cardboard box approximately to the front center part of the heated. Move the X-axis all the way to the front side of the printer, and move the extruder approximately to the center of the X-axis.
Open the dwarf-cover-door to access the Dwarf board.
Check that all the connectors are secure in their sockets, according to the picture below. Focus on the thermistor and heater cables. Though unlikely, rough shipping or handling could have shaken the heater or thermistor cables loose.
The table below describes the correct resistance for each part, as well as what scale you should set your meter to. The hotend thermistor is rated to be 100 kΩ at 25 °C. To be realistic, with a varying temperature between 20 °C and 30 °C, you can expect a wider range of readings (~80-125 kΩ).
There can be micro-fractures in the cable, breaking the connections only at certain positions. Therefore, try to move the cable a bit while doing the measurement and see if the value changes drastically.
[80 kΩ - 125 kΩ]
[12.3 Ω - 15.1 Ω]