Bambu Lab HMS error code 0300-0200-0001-0001 flags a fault in the hotend temperature sensing circuit. The 0300-0200 subgroup covers the nozzle and hotend thermal system, and this specific error indicates the thermistor circuit appears open — meaning the printer is reading no valid temperature signal from the nozzle at all.

What the Error Means

An open-circuit thermistor condition means the resistance in the sensing circuit is effectively infinite, which the firmware interprets as a broken wire or disconnected sensor. This is treated as a critical safety fault because operating a hotend without temperature feedback risks uncontrolled heating. The printer will refuse to heat the nozzle until the fault is cleared.

Step-by-Step Fix

1. Power off the printer immediately and allow the hotend to cool completely — at least 10 minutes — before touching the toolhead.
2. Open the toolhead cover according to your printer model’s disassembly guide. The thermistor is a small two-wire component embedded in or adjacent to the heater block.
3. Inspect the thermistor wires for visible breaks, kinks, or burn marks near the heater block where heat cycling can cause wire fatigue.
4. Trace the thermistor cable up through the toolhead to its connector on the toolhead PCB. Unplug and reseat the connector firmly.
5. If the wires appear intact, use a multimeter set to resistance mode: measure across the thermistor pins at the connector. A functional NTC thermistor at room temperature typically reads 80–120 kΩ. An open reading (OL) confirms the thermistor has failed.
6. Replace the thermistor with the correct Bambu Lab spare. Ensure the new thermistor is fully seated in the heater block recess.
7. Reassemble the toolhead, power on, and run a preheat cycle to confirm the temperature reads correctly.

Parts Required

• Hotend thermistor (NTC 100K, Bambu Lab toolhead-compatible)
• Multimeter (for diagnostic continuity check)
• Hex drivers for toolhead disassembly
• Kapton tape (to secure wiring after reassembly if needed)

If the error persists after thermistor replacement, contact Bambu Lab support — the toolhead PCB connector or wiring harness may be damaged.