This issue can be preceded by the error Err: EXTR. FAN ERROR. Before we start checking your Hotend fan, power down and unplug your USB cable if it's connected. Open the electronics covering and examine the Mini RAMBo electronics wiring (MK2S, MK2.5, MK2.5S) or Einsy RAMBo electronics wiring (MK3/MK3S/MK3S+).
You can easily test the nozzle Hotend fan function from the LCD menu Preheat - PLA. From when the Hotend temperature reaches 50 °C, the fan should immediately spin up.
On MK2.5S/MK3S - From the LCD menu - Support - Extruder Info, you can find the RPM reading of the 3-wire fans that support this. The expected range should be from ~3500 - 4200 RPM on both fans, when run at full speed. Only the print fan speed can be adjusted by the user.
Hotend cooling fan cools down the top part of the Hotend (heatsink) from the heat which is transferred from the heater block. It always starts up when Hotend temperature reading reaches 50°C and spins for the whole duration of the print! If the fan is not functioning, you can expect the nozzle to jam after the first few layers due to heat creep.
|Correct: Sticker facing inwards
|NOT Correct: Sticker facing outwards