Contact tips
The tip carries the current into the wire. As the bore wears oval, contact gets erratic: you'll see an unstable arc, spatter, wire sticking or burn-back.
Change tips as soon as the arc goes unstable. They're the cheapest part on the torch, and a worn one costs you far more in grinding and rework.
Nozzles and diffusers
Spatter build-up in the nozzle disturbs the gas shield, which gives you porosity. Clean nozzles regularly and use anti-spatter.
A blocked or damaged diffuser does the same. Check it whenever you change the tip.
Liners
The liner guides the wire through the torch cable. Dirt, swarf and kinks make feeding rough and cause bird-nesting at the drive rolls.
If wire feed feels rough with a new tip fitted, suspect the liner. Use the right type and size for the wire: steel liners for steel wire, and plastic liners for aluminium.
Drive rolls
Drive rolls must match the wire size and type: V-groove for solid steel, U-groove for aluminium, knurled for flux-cored. Worn grooves slip, and too much pressure deforms the wire and makes feeding worse.
If the machine's wire feed speed is calibrated but the arc still isn't right, worn drive rolls are often why.
Why it matters for compliance
A machine can pass calibration and still weld badly through worn consumables. That's why we check torches and wire feed when we calibrate, and stock the consumables for the machines we look after.