HMS 1A00-0100-0002-0004: Hotend Swap Check Failed, a Hotend Is Already Installed
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HMS 1A00-0100-0002-0004 means the pre-swap check found a hotend already sitting in slot 1 when it expected that slot to be free. The machine stops rather than trying to put one hotend on top of another, which is exactly the behaviour you want from a tool changer.
This is the opposite of the empty-slot fault, and it arrives at the same point in the sequence: before anything has moved.
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Why the rack can be surprised
The machine tracks which slot holds what. Move a hotend by hand between prints, swap two of them around, or interrupt a previous swap partway, and its record no longer matches the rack. The next job that wants to place a hotend into slot 1 then finds it occupied.
Clear the slot, then let it re-check
Take the unexpected hotend out, note which slot it came from, and store it where it belongs. Then continue: the check re-runs and should pass. Do not force a placement, because a tool changer pressing a hotend into an occupied bay is how mounts get bent.
When the rack is simply full
If every slot holds something and the plate wants to add another, the honest message is a different one: HMS 1A00-1300-0002-0001 tells you the rack is fully occupied. Getting that code instead means nothing is misplaced; you just have more hotends than bays.
What HMS 1A00-0100-0002-0004 protects
A collision between a mounted hotend and one being placed damages the expensive half of the machine, not the cheap half. The check exists because the recovery from that is a part order rather than a tidy-up. The hardware context is in 3D printing hotends.
The habit that prevents it
Return hotends to the slot they came from, and finish a swap before powering down. Half-completed swaps are the most common way the record drifts, and the power-loss version of that is HMS 1A00-2000-0002-0001. The empty-slot mirror of this fault is HMS 1A00-0100-0002-0003.
HMS 1A00-0100-0002-0004 FAQ
Can I just move the hotend myself?
Yes, with the machine idle. Remove it cleanly rather than levering, and let the check run again.
Why does it always say slot 1?
Because that is the slot the current operation was targeting. Another job may name a different bay.
Does it mean two hotends are the same type?
Not necessarily. The check is about occupancy, not about which nozzle size is where.
Will a firmware update help?
Not usually. This is a physical mismatch rather than a software one, which is why clearing the slot fixes it.
Collection fails as readily as placement. HMS 1A00-0100-0002-0032 means the toolhead could not confirm the hotend on the rack, and Bambu names debris in the mounting slot.
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