Your printer just halted with "Err: MINTEMP", "MAXTEMP triggered", "Thermistor disconnected", or one of dozens of close cousins. These errors all mean the same general thing: the firmware can't trust the temperature it's reading, so it shut the heater off to keep your printer from burning down. Here's what each one means and the systematic way to fix it.
MINTEMP almost always means a disconnected or broken thermistor wire near the hotend. Wiggle the hotend wires while watching the temperature reading; the temp will jump. MAXTEMP usually means a shorted thermistor (broken wire shorting against the heater block) or, more rarely, a stuck-on heater. Power off immediately if you suspect a stuck heater. Never bypass these errors — they exist to prevent fires.
A thermistor is a temperature-sensitive resistor. Its resistance drops as it heats up — at room temperature it's around 100 kOhm; at 200 °C it drops to about 1 kOhm. The firmware reads the voltage across the thermistor and converts that to a temperature using a calibration table. So an "error" can be: cable broken (infinite resistance → near-zero temp reading → MINTEMP), cable shorted (zero resistance → very high temp reading → MAXTEMP), or some intermediate fault.
Marlin's default MINTEMP threshold is 5–15 °C . Prusa MK3/MK4 use similar values; Bambu and Klipper-based printers use comparable but firmware-specific thresholds. If the firmware reads a temperature below this threshold, it concludes the thermistor is disconnected.
Marlin's default MAXTEMP for hotends is 305 °C ; for beds it's 125 °C . Prusa firmware uses similar values. If temperatures exceed these, the firmware kills power to the heater.
Cut mains power at the switch or unplug. Don't try to reset the printer or interact with the LCD. A true thermal runaway is the rare but real failure mode that thermistor errors exist to prevent.
If you replace your hotend and don't update the thermistor type in the firmware, you'll get persistent temperature errors. Marlin: #define TEMP_SENSOR_0 in Configuration.h . Klipper: sensor_type: in printer.cfg . Bambu/Prusa: the printer detects the type automatically in most cases.