Anycubic - when inserting filament, the ACE Pro begins to feed but then immediately stops

Issue

When inserting filament into your ACE Pro, it begins to feed, but then immediately stops. If you jiggle the filament, it feeds a little further. The ACE Pro display on the screen does not show any filament is loaded. Article applies to Kobra S1 Combo (TL5210) and Kobra 3 Combo (TL5200) printers.

Cause

The filament sensor is not registering due to too much wear or spring tension in the pawl that the filament pushes into an optical sensor. See this thread.

Fix

Anycubic's overview is here.

Unplug the ACE Pro power and data cables. 

Unscrew the two 2.5mm Hex screws on either side of the silver cover. 

Pop the cover out - it has clips on the front edge that need to be engaged first when you reinstall it.

Undo the two screws holding the filament sensor bar and lift it out - don't drop the screws into the ACE.

Unplug the cable gently from the affected port.

Undo the single screw that holds the PCB to the front of the affected port.

Lift the PCB away from the sensor bar, and check the pawl that's inside it for free movement. Poke a piece of filament through the hole on top to see how it works - it should pivot on the two rods either side of the spring and move towards the PCB when the filament is there. It interrupts an optical sensor when it does this. 

If this is a new unit, a replacement filament sensor hub is available as Cat # 001907. A complete ACE Pro is TL5208.

You can also trim one loop from the spring with scissors, and reassemble the unit to see if this is enough to fix it. 

If the unit has worn out the pawl, or the pawl or spring has been lost/damaged, you can print a replacement sensor support that deletes the pawl and uses the filament itself to complete the detection loop - the only caveat is that you may find it will not detect some transparent filaments. See here. You can still print with those manually via the spool holder, just as you would with non-ACE-compatible filament like TPU.