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Selector cannot home #04115 (MMU)

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Last updated a year ago
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What happened?

Before it can align with any of the filament positions, the selector on the MMU unit needs to be “homed” to determine its position. For that, the selector moves from one end stop to the other while measuring the distance between them. The selector will home after the unit starts up, has been reset, or after another error occurred.

In case the Selector can't find the first homing endstop, or the distance between the endstops differs from the expected value, the printer will show this error.

Distance between the Selector endstops: 75mm
Allowed deviation: 3mm

How to fix it?

The selector might not move due to obstacles, debris, or remaining filament in its way. Before proceeding, make sure that the path of the selector is clean, and that any filament that has been cut by the selector blade is removed.

Clean also any filament that might be under the selector in the pulley body.

Clearing the Selector

Open the front Festo fitting to check the Selector.

If there is filament sticking out from the front PTFE tube, or sticking out from the MMU unit into the selector, take an Allen key to push the filament back.

If the selector has tried to move with the filament still inside it, turn the printer off, and manually turn the motor threaded rod until it aligns with the pulley body and you can pull the filament from the back of the MMU unit or push from the front of the selector with an Allen key. After that, adjust the FINDA height.


In case the Selector is clean from filaments, check the following items in the MMU unit assembly.

  • Verify if the SuperFINDA sensor reads correctly.
  • The trapezoid nut on the selector motor might be loose or offset to one side. Remake the Selector motor installation.
  • The bushing tubes should be flush with the body of the Selector.
Both bushing tubes and the trapezoid nut are self-lubricating. Do not apply any lubricant to the trapezoid nut.
  • The selector motor must not be loose on the MMU unit body. The top corner screw should be tightened slightly looser compared to the other two. The two screws should be tightened evenly. Remake the Selector motor installation.
  • In case you have printed the parts yourself, check if the printed parts do not have any surfaces that are lifted.
  • Especially the first time you are using the MMU unit, verify that the motors are connected in the correct order.
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