MINTEMP, MAXTEMP & thermistor errors

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.

How thermistors work (briefly)

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.

MINTEMP — what it means and how to fix it

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.

Causes (in order of likelihood)

1. Wiring fault near the hotend. The wire flexes thousands of times per print and eventually breaks at a stress point — usually right where it enters the heater block or where it exits the cable chain.
2. Loose connector at the board. Re-seat the T0/HEATER_T0/thermistor connector at the mainboard.
3. Broken thermistor. The bead itself has cracked or burned out.
4. Cold environment. Garage in winter; the thermistor truly is reading 4 °C. Rare but real.
5. Wrong thermistor type in firmware. After a hotend swap (NTC 100K vs PT1000 vs PT100), the firmware calibration table is wrong.

How to diagnose

1. Power on the printer cold. Read the hotend temperature on the screen.
2. If it shows "0°C" or "ND" (not detected) — cable is fully disconnected. Check the connector at the mainboard.
3. If it shows ambient (~20 °C) but jumps to MINTEMP after a few minutes of moving — intermittent cable fault. Gently move the hotend through its full XY range while watching the temperature reading. It'll glitch when you hit the bad spot.
4. If the wire looks fine but the reading is wrong, swap the thermistor itself (cheap, 5 minutes to replace on most printers).

MAXTEMP — what it means and how to fix it

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.

Causes (in order of likelihood)

1. Shorted thermistor wire. The insulation has melted or chafed and the bare wire is touching the heater block or another conductor, dropping resistance and faking a high reading. Most common cause.
2. Loose thermistor bead. The thermistor bead has come out of its hole; with no thermal connection to the block, it heats up uncontrolled by direct contact with the cartridge or convection.
3. Actual runaway: stuck heater MOSFET (rare on quality boards). Power off immediately if you suspect this.
4. Wrong thermistor type in firmware after a hotend swap.
5. Wrong slicer/printer combination — sliced for a high-temp printer and trying to print at 300 °C on a stock Ender 3 with PTFE in the hotend (which also off-gases above 250).

Thermistor types in modern printers

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.

Firmware-specific error variants

Marlin (Ender, Sovol, Anycubic, many others)

• Err: MINTEMP — below minimum threshold.
• Err: MAXTEMP — above maximum threshold.
• Heating failed, system stopped — heater wasn't reaching target fast enough.
• Thermal runaway — temperature drifted unexpectedly during a print.

Prusa Buddy (MK4, CORE One, XL)

• "MINTEMP HOTEND" / "MINTEMP BED" — under-temp.
• "MAXTEMP HOTEND" / "MAXTEMP BED" — over-temp.
• "Thermistor disconnected" — clear diagnostic name; same root cause as MINTEMP.

Klipper (Voron, Sonic Pad, RatRig)

• "ADC out of range" — equivalent to MINTEMP or MAXTEMP. Klipper checks min/max temperature in printer.cfg.
• "Heater is shutdown" — safety triggered; you must FIRMWARE_RESTART after fixing.

Bambu Lab (X1, P1, A1)

• TE2509: Unable to obtain nozzle temperature, thermistor may be disconnected.
• TE2515: Nozzle temperature exceeds limit, thermistor may be short-circuited.
• TE2111: Nozzle temperature below minimum extrusion temperature (often a tied error after extruder cooling unexpectedly).
• TE2564: Nozzle is not heating as expected.
• CB2510 / CB2516: Hotbed thermistor open or short circuit.

Prevention

• Inspect hotend wiring monthly for chafing and fatigue.
• Replace the silicone sock if torn — it protects the thermistor wires from molten plastic contact.
• Strain-relieve the cable chain. Don't let wires flex at the same point every time.
• Use ferrules on stranded wires at the mainboard terminal.
• Buy spares. Thermistors cost $3–$10; have one on the shelf so a printer down isn't a printer down for days.

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