HMS 0300-0900-0001-0001: extruder motor open circuit
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The controller sees an open circuit where the extruder motor should be. Check the connector at the toolhead before condemning the motor: these leads sit in the most-flexed cable bundle on the machine.
A stepper motor is a pair of coils. The driver expects to measure a sensible resistance across each one, typically a few ohm, and an open reading means the loop is broken somewhere between the driver and the windings.
Affected machines: A2L, H2, H2S, P2S, X1 series.
The cable bundle moves more than the motor does
The extruder motor itself is bolted down and barely vibrates. Its wiring, on the other hand, rides with the toolhead on every single move of every print. That is where conductors fatigue. Check continuity with the toolhead parked at several different positions, because a broken strand often makes contact in one position and not another.
Extruder does not turn at all and the code appears as soon as the printer tries to move filament
Broken conductor in the toolhead loom, or a connector that has worked loose
- Power off at the wall.
- Unplug the motor and measure across each coil pair. A few ohm on each is normal; open on either confirms a break.
- If the motor measures correctly, move the fault upstream to the loom and connector.
- Repeat the continuity check with the toolhead moved to each corner of the bed to catch an intermittent break.
Never unplug or reconnect a stepper motor while the printer is powered. Doing so can put a voltage spike into the driver and turn a broken wire into a dead mainboard.
Open circuit and short circuit are different faults
A companion code reports a short instead, and it usually does mean the motor has failed internally. This one, reading open, points at wiring far more often than at the motor. That difference is worth reading carefully before you order a replacement.
Wrong code? The full Bambu HMS code index lists all 588 codes with the subsystem each one points to and which printers show it.
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