Heating failed or thermal runaway error

The printer stops with an error like 'heating failed', 'thermal runaway' or 'MINTEMP', or the temperature drops during a print.

Also described as thermal runaway, heating failed, mintemp, maxtemp, temperature dropping, printer halted, won't heat up

Try these in order

Most likely and quickest to check first. Change one thing at a time so you know what fixed it.

Part cooling fan blowing on the heater block
When the fan speeds up, the heater can't keep up and the safety check stops the printer.
- Fit a silicone sock on the heater block.
- Run a PID tune with the fan on (M303 E0 S210 C8, then M500 in Marlin).
Loose heater or thermistor wire
Readings that jump around or drop to room temperature point to a wire or connector.
- With the printer off and cold, check the thermistor and heater cartridge are seated and their wires and plugs aren't broken.
- Replace a damaged thermistor; they're cheap.
Bed or enclosure too cold
- Keep the printer away from cold draughts, and let the bed heat fully before starting.