A hand planer that no longer leaves a clean finish is the second most common joinery tool through our door for spares, behind only the angle grinder. The two parts that fail are the blades and the toothed drive belt, and the symptoms are different enough that you can usually tell which without taking the tool apart.
Blades go first on most tools. Symptoms are torn rather than sheared fibres on softwood, a ridged finish from a chipped blade, snipe at the start of the cut, or the smell of burnt resin on long passes. Replacement planer blades come in three formats: reversible double-edged HSS, single-edged TCT, and disposable double-edged HSS designed to flip rather than sharpen. Length is the critical dimension — 82mm for most handheld planers, with smaller and larger blades for trim planers and bench planers. Whatever you fit, set both blades parallel to the rear sole with a straight edge before running the tool.
Belts go second. The symptom is a planer that runs normally on no-load but loses bite immediately under cut — the motor spins but the cutter head doesn't keep up. Open the side cover and you'll usually see a row of stripped teeth or a belt that has gone slack on one side. Planer drive belts are a direct fitment by model and take ten minutes to swap once the cover is off. The Bosch Green PHO range and the Makita 1900B are the most common belt swaps we see.
The third thing to check, if blades and belts both look fine, is the brushes. A planer that has lost power overall, or that runs intermittently, points to carbon brushes rather than belts or blades. The brand-specific routes — Makita, Bosch, DeWalt — cover the planers as well as everything else.
If the planer also shows wobble at the cutter head, the spindle bearings are tired. They are inexpensive and a sensible swap while the tool is open: see bearings and mechanical components for the standard sealed bearings used across the major brands.