The hotend or bed won't heat up, or takes too long

Symptom

You set a temperature and the hotend or bed doesn't rise, rises very slowly or stays far from the target; often the printer ends up showing a preheat or heating error. Sometimes the displayed temperature doesn't change even though the part does get hot.

Likely causes, from most to least likely

1. A cold room or airflow cooling the block or bed: a fan blowing on the block, a window or air conditioning.
2. A loose or badly seated heater or thermistor connector on the board or at the print head.
3. A wire broken inside, especially in the sections that move with the print head or bed.
4. A blown fuse on the board, on printers that have them.
5. A faulty heater (cartridge or bed) or one with too high a resistance.
6. A faulty, loose or displaced thermistor: the firmware doesn't see the temperature rise even though the heater works.

Checks to tell which one it is

• Hair dryer test: blow hot air at the hotend or bed and watch the display. If the temperature rises, the thermistor works and the fault is in the heater or its connection; if nothing changes, the thermistor is faulty.
• With the printer off and unplugged, measure the thermistor with a multimeter at its connector: if it shows no resistance, the wire is broken. Check the expected value in your manufacturer's documentation.
• While it heats, move the print head or bed from end to end and move the cable bundle: if the reading jumps by tens of degrees or drops to zero, there's an internal break.
• Check that the connectors' latches are engaged, that the bed thermistor is still firmly attached to the bed, and that the hotend thermistor wire isn't damaged where it enters the block.
• If you built or configured the board with Klipper: set the extruder to 50 °C; it should start rising within about 30 seconds. If it doesn't, check heater_pin in the configuration.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Get rid of the cold

   Move the printer away from windows, drafts and air conditioning, and make sure no fan blows straight at the heater block. In a very cold room, heating can be slow or fail.

   Check: The temperature rises steadily to the target.

   If that does not fix it: Check the connections.

2. Turn off and check the connections

   Turn off and unplug the printer and let it cool. Reseat the heater and thermistor connectors on the board and at the print head until the latch clicks, and look for stripped or crushed wires.

   Check: When switched back on, the temperature rises when you ask it to.

   If that does not fix it: Check the fuses.

3. Check the fuses

   If your board has fuses, check with the printer unplugged whether one is blown and replace it only with one of the same type and rating. If it blows again, stop testing and contact support.

   Check: The heater heats again.

   If that does not fix it: Find out whether the thermistor or the heater is at fault.

4. Find out whether the thermistor or the heater is at fault

   Do the hair dryer test. If the displayed temperature rises, the thermistor is fine and the fault is in the heater or its wiring; if it doesn't change, the thermistor is bad. If you can use a multimeter, measure both parts as well.

   Check: You know which part is faulty.

   If that does not fix it: Ask your manufacturer's support for help.

5. Replace the part and recalibrate

   Replace the heater cartridge, thermistor or its cable with compatible spares following your manufacturer's guide. Then recalibrate the PID (in Marlin, with M303; in Klipper, with PID_CALIBRATE and SAVE_CONFIG).

   Check: It reaches the set temperature and holds it steadily.

   If that does not fix it: Contact support: the board output may be faulty.

Prevention

• Check heater and thermistor connectors during routine maintenance, with the printer unplugged.
• Check that the sleeve protecting the print head cable bundle is still in place.
• Recalibrate the PID after changing the hotend, heater or thermistor.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Preheat error · Prusa Research
• MINTEMP error and MINTEMP BED · Prusa Research
• Configuring Marlin (Thermal protection and PID) · Marlin
• Configuration checks (Verify heaters, Calibrate PID settings) · Klipper