When a MIG gun stops feeding wire, it's almost never the machine's electronics — it's the mechanical path the wire travels, from the spool to the contact tip. The same handful of causes account for the large majority of "won't feed" and birdnesting calls, and you can isolate which one you have in about two minutes with a couple of tests below. Work them in order: tension, then groove, then the liner, then the tip. If you're on a specific welder, the machine pages at the bottom give the exact tip, liner, and drive-roll part for that gun.
Straighten the gun out, hold the trigger, and watch. If the wire stalls but the drive motor still turns, the rolls are slipping — tension is too low. If the spool keeps spinning and wire piles up into a 'birdnest' between the spool and the rolls, the wire jammed somewhere downstream and the rolls kept shoving. Too-tight tension is just as bad: it shaves the wire and packs the liner with fine debris.
Back the tension knob fully off, feed the wire against a block of wood or scrap, then tighten only until the wire stops slipping — and add about a half turn past that point, no more. That is the whole adjustment; people routinely over-tighten and cause the very birdnesting they're trying to stop.
Read the size stamped on the face of the drive roll and compare it to your wire; then look at the groove profile itself.
Match the groove to the wire: a smooth V-groove for solid steel wire, a U-groove for soft aluminum, and a knurled (serrated) V-groove for flux-cored wire. Most rolls are double-sided (e.g. .030 / .035) — make sure the side facing the wire is the one stamped for your size. A .035 groove running .024 wire lets the wire bounce side-to-side and tangle.
Disengage the drive rolls and pull the wire through the gun by hand. It should pull with light, steady effort. If it drags hard or won't move, the liner is your restriction — often packed with fine copper/steel dust shaved off by over-tight rolls.
Blow the liner out with low-pressure compressed air. If it's kinked, gouged, or full of debris, replace it — sized to the wire and cut to the correct length (a liner left too long or trimmed too short feeds badly). On a working machine, plan to replace the liner about once a year.
Pull the tip and sight through the bore. It should be round and sized to your wire. An arc-marked, oval, or oversized bore drags the wire and causes intermittent feed and burnback.
Replace it with a tip whose inner diameter matches the wire (a .035 tip for .035 wire). The contact tip is the cheapest consumable in the gun and the most common quiet cause of a feed that 'just stopped' — keep a handful of spares.
Wire feeds in bursts or stalls under load. Check how hard the spool is to turn by hand.
Loosen the spool-hub nut until the spool turns with light resistance — just enough that it stops coasting and overrunning when you release the trigger, but not so much that the drive rolls have to fight it. An over-braked spool starves the rolls; an under-braked one overruns and births a loose tangle.
Unspool a foot or two and look for rust, kinks, or a cast so tight the wire won't lie roughly flat. Rust and surface debris are abrasive and bind in the liner and tip.
Cut back to clean, bright wire; if the spool surface is rusted through, replace it. Store partial spools sealed in a bag with desiccant, not open on the machine in a humid shop.