HMS 0300-0900-0001-0002: extruder motor short circuit
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The driver sees a short across the extruder motor. Where the open-circuit version of this fault is nearly always wiring, a short usually means the motor itself has failed internally. Stop printing rather than retrying.
A stepper motor is two coils, each normally a few ohm. The driver watches the current it has to supply to reach a commanded step. If that current implies far less resistance than a healthy coil, the driver shuts down to protect itself.
Affected machines: A2L, H2, H2S, P2S, X1 series.
Why a short points at the motor and an open points at the cable
The two failure modes have different physical causes. A break happens where a conductor flexes, which is in the loom. A short happens where insulation between adjacent turns breaks down, which is inside the winding, and that is driven by heat rather than movement. So the same subsystem produces two codes with two quite different answers, and the code you have is the useful clue.
Extruder does not move and the printer stops immediately, sometimes after a period of the motor running unusually hot
Shorted turn inside the motor winding, or a crushed lead shorting two phases together
- Power off at the wall and let everything cool.
- Unplug the motor and measure each coil pair. A few ohm on each is healthy; near zero on either confirms a shorted winding.
- Measure between the two coils. They should be isolated from each other; continuity there means the phases are shorted.
- Inspect the lead where it enters the motor body for crushing or melted insulation before you conclude the motor is at fault.
Do not power the printer back on to test repeatedly. A shorted winding puts the driver MOSFETs under stress every time, and a motor fault that costs one part can become a mainboard fault that costs several.
Check the driver before fitting a new motor
A short can damage the driver that was feeding it. If a replacement motor produces the same code immediately, the fault has moved upstream to the board. Test the new motor off the machine first so you know its resistance is good, and you will be able to tell the two situations apart.
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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