Finding it
Leave the tester above open and do not touch the keyboard. Any cap that shows as pressed is physically down. After fifteen seconds it is marked as stuck and starts pulsing, because no one holds a key that long on purpose.
This is the fastest way to identify the culprit, because the symptoms rarely point at the key. A stuck Shift makes everything uppercase. A stuck Control turns ordinary letters into shortcuts and makes a machine seem possessed. A stuck arrow key scrolls documents on its own.
Physical causes
- Debris under the cap, which is by far the most common. Crumbs, hair and dust stop the stem returning.
- Spilled liquid, especially anything sugary, which dries into a glue. Sugary drinks are worse than water by a wide margin.
- A keycap fouling its neighbour or the case, often after the cap was removed and reseated slightly off.
- A bent or unlubricated stabiliser on a large key such as the space bar, Enter or Shift.
- A failed switch spring, which is rare but does happen.
Fixing it
Start by pressing the key firmly a dozen times. A surprising share of stuck keys free themselves, because the obstruction is loose debris rather than anything adhesive.
If that fails, pull the keycap. On a mechanical keyboard a keycap puller costs very little and removes the risk of snapping a stem. On a laptop, look up the specific removal method for that model first, because scissor clips break easily and are not always replaceable on their own.
With the cap off, compressed air and a soft brush clear most debris. For dried liquid, a cotton bud with isopropyl alcohol works and evaporates cleanly. Let it dry completely before reconnecting power.
If a key is stuck because of a spill that has spread, stop, disconnect the keyboard, and deal with the spill properly before testing anything further.
When nothing is physically stuck
If the tester shows nothing held down but characters still repeat, the fault is more likely to be chatter or a software setting than a stuck key. Windows key repeat settings, Sticky Keys and remapping utilities can all produce the same symptom.
A keyboard that reports a modifier as held immediately after connecting, then clears when you tap that modifier once, is usually a lost key-up event rather than a hardware fault. It is common after switching between machines with a shared keyboard.