HMS 0300-0700-0001-0001: Motor-B open circuit
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HMS 0300-0700-0001-0001 means Motor-B reads as an open circuit: the board sends current and sees nothing come back. On a CoreXY machine Motor-B is one of the two belt motors, so the printer will not home and will not move the toolhead correctly. The cause is a broken connection far more often than a dead motor.
Affected machines: A2L and P2S. Motor-B is the right-hand motor of the CoreXY pair. Because both motors drive both axes through a crossed belt path, losing one does not lose one axis: it breaks all planned motion in X and Y together.
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Why the connector fails before the motor
A stepper winding is a coil of enamelled wire in a sealed can. It rarely goes open on its own. The connection between that can and the board, on the other hand, is a crimp in a plug that lives in a moving loom, and that is the part that fails: a pin that has backed out of its housing, a crimp that was never fully closed at the factory, or a wire that has fatigued where it flexes.
That is the useful thing about this code. It tells you a circuit is broken, not where. Treating it as a dead motor and ordering a part first is the expensive way round.
Printer refuses to home, or stops with Motor-B open circuit
Broken connection in the motor circuit, occasionally a failed winding
- Power the printer off at the wall and wait for the display to go dark.
- Unplug Motor-B at the board end and inspect both halves of the connector. Look for a pin sitting lower than its neighbours.
- Set a multimeter to resistance and measure across each coil pair at the motor plug. A healthy stepper reads a low, even resistance on both coils, typically a few ohms.
- If both coils read healthy at the motor, the break is in the cable or the board connector. Reseat the plug and repeat the reading at the board end.
- If one coil reads open at the motor itself, the motor has failed. Fit the replacement, keeping the same pulley height on the shaft.
When you refit a motor, pulley height matters as much as the wiring. A pulley set even a millimetre off its original position makes the belt ride against a flange, which shows up later as a whine and uneven wear rather than as an error code.
The companion fault on the other motor is Motor-A open circuit, and it is diagnosed the same way. Our page on CoreXY printers explains why one motor failing breaks both axes, and the troubleshooting index covers the rest.
HMS 0300-0700-0001-0001 FAQ
Can I keep printing with this code?
No. The machine cannot position the toolhead reliably, and forcing it risks driving the head into the frame.
What resistance should the motor read?
A few ohms across each coil pair, and the two pairs should read within a fraction of an ohm of each other. Infinity means open.
Is Motor-B the X or the Y motor?
Neither on its own. On CoreXY both motors move both axes, which is why one failure stops all planned motion.
Does the same motor fit Motor-A?
On these machines the two are the same part, so a spare covers either position.
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