HMS 0300-0600-0001-0001: motor A open circuit
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Motor A is reading open circuit. On a CoreXY printer the two motors work as a pair, so a single dead motor does not lose one axis, it loses controlled movement in both. Check the connector at the mainboard first.
CoreXY machines drive the toolhead with two motors, conventionally called A and B, coupled through a crossed belt path. Neither motor owns an axis. Moving in X requires both to turn together, and moving in Y requires them to turn in opposition, which is why one failed motor stops everything rather than half of it.
Affected machines: A2L, H2, P2S, X1 series.
The connector at the board, not the one at the motor
These motors sit still in the frame, so their own leads see very little movement. The connection that fails is usually at the mainboard end, where the harness is routed round a corner and pulled taut during assembly. A crimp under constant tension will back out of its housing over months of heating and cooling.
Toolhead does not move, or the printer stops during homing before any motion happens
Backed-out crimp or broken conductor in the motor harness, occasionally a failed winding
- Power off at the wall.
- Unplug the motor A connector at the board and measure each coil pair. A few ohm on each is healthy.
- If the motor reads correctly, the break is in the harness. Tug each wire individually behind the connector housing.
- Compare against the motor B connector. Two identical parts side by side make an abnormal reading obvious.
Never plug or unplug a stepper motor with the printer powered on. The collapsing field in the winding can put a spike into the driver, which is a good way to turn a loose connector into a replacement mainboard.
Layer shifts are the warning that precedes this
A motor circuit that is intermittent rather than fully open usually shows up as layer shifts first: the toolhead loses steps, the print offsets, and the machine carries on unaware. If you had unexplained shifts before this code appeared, they were the same fault at an earlier stage.
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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