How to Fix HMS 0300-0300-0002-0002: Hotend Cooling Fan Speed Error

Bambu Lab HMS error code 0300-0300-0002-0002 flags an issue with the hotend cooling fan speed. While the 0001-0001 variant covers complete fan failure, suffix 0002-0002 in the 0300-0300 fan subgroup typically indicates the fan is running but its measured RPM is significantly below target — pointing to a degraded motor bearing or a partial obstruction reducing airflow.

What the Error Means

The hotend cooling fan (also called the cold-end or heatsink fan) is critical for preventing heat creep — the upward migration of heat from the heater block into the heat break and cold zone. Unlike the part cooling fan, this fan should run continuously whenever the hotend is hot. Reduced fan speed can cause heat creep leading to jams and heat-related damage in the PTFE lining. The firmware raises this error when the tachometer reports RPM below a safe threshold.

Step-by-Step Fix

  1. Power off the printer and cool the hotend before touching the toolhead.
  2. Inspect the hotend cooling fan (typically located at the rear or side of the toolhead, blowing through the heatsink fins). Look for dust buildup packing the heatsink fins, filament fibers caught in the fan blades, or debris lodged in the housing.
  3. Use compressed air to blow out the heatsink fins and fan housing. Clean from inside the heatsink outward to avoid pushing debris deeper.
  4. Manually spin the fan to check for bearing drag. A fan with worn bearings will feel rough or wobble on its axis.
  5. Reseat the fan connector at the toolhead PCB.
  6. Power on and run the fan diagnostics from the printer menu to check whether the fan reaches its target RPM after cleaning.
  7. If the fan still underperforms after cleaning and reseating, replace it — a degraded bearing will only worsen over time and risks a full stall.

Parts Required

  • Hotend cooling fan (heatsink fan, model-specific for your Bambu Lab printer)
  • Compressed air canister or air blower
  • Small screwdrivers for fan access panel

If the error continues after replacement, contact Bambu Lab support to check the toolhead PCB fan control circuit.