From one day to another, my printer started severely underextruding. A quick search unveiled, that this is a common problem with Ender 3's. It seems like the extruder's spring sometimes just loses its tension and that the extruder then just isn't able to grab the filament anymore.

There are some quick fixes like putting a washer between the spring and its mount or even printing prefitted parts. This fix worked very well for me - for about two minutes, when the extruder just stopped working at all.

Disassembling the whole thing surfaced the underlying problem: The whole extruder arm just gave up the ghost und broke, unrepairable. The part holding the guiding wheel broke and could not clamp the filament with enough tension anymore. Time for a replacement. Not a printed one.

Fortunately, it seems that this is one of the most common parts breaking with this printer, so there are various replacements available from amazon. Also, they are made from aluminum, so I think this one will last a bit longer.

It works perfectly well. It's coated red, which makes the printer itself faster, too. Obviously.