Prusa 3D Printer Error Code 4: How to Fix It (MAXTEMP Hotend)

Q: What causes Prusa Error Code 4 MAXTEMP?

Prusa Error Code 4 is almost always caused by a faulty, loose, or disconnected thermistor in the hotend. The thermistor is the tiny sensor that tells the printer the current hotend temperature. If it falls out, breaks, or loses connection, the firmware reads an abnormally high temperature and shuts down as a safety measure. Less commonly, it can be caused by a damaged thermistor cable or a bad connection at the mainboard.

Q: Is it safe to keep printing after Error Code 4 appears?

No. Error Code 4 is a thermal safety shutdown and you should never attempt to bypass or ignore it. The error exists to prevent uncontrolled heating, which can cause fires, damage your printer, and ruin prints. Always investigate and resolve the root cause before resuming printing.

Q: How do I know if my Prusa thermistor needs replacing?

Use a multimeter set to resistance mode and measure the thermistor with the printer unplugged and cooled to room temperature. A healthy NTC 100k thermistor should read approximately 100,000 Ohms (100kΩ) at 25°C. If you get a reading of 0 Ohms (short), infinite resistance (open circuit/OL), or a wildly incorrect value, the thermistor needs to be replaced.

Q: Can a Prusa MAXTEMP error be caused by the heater cartridge?

Yes, though less commonly than a thermistor fault. If the heater cartridge is shorted or malfunctioning, it can heat uncontrollably and trigger a MAXTEMP error before the firmware can respond. You can test the heater cartridge resistance with a multimeter — a typical 40W Prusa heater cartridge should read around 30–40 Ohms. An open circuit (OL) or short (near 0 Ohms) indicates a failed cartridge that must be replaced.

Q: How do I prevent Prusa Error Code 4 from happening again?

The most effective prevention is to periodically check that your thermistor is securely seated in the heater block with its retention screw properly tightened, and inspect the thermistor cable for wear especially near the cable chain or extruder where bending stress accumulates. Also keep Prusa firmware updated, as improvements to temperature monitoring are often included in updates. Avoid crashing the printhead into the bed, which can dislodge the thermistor.

Abstract

Prusa Error Code 4 is a MAXTEMP hotend error, meaning the printer's firmware detected that the hotend temperature exceeded a safe maximum threshold — or received an abnormally high reading from the thermistor. This is a protective shutdown designed to prevent fire, filament damage, or hardware failure. The most common causes are a faulty or disconnected thermistor, a damaged thermistor cable, or a runaway heating situation where the hotend heater cannot be properly regulated.