What you're looking at
A key lights up while you hold it down and stays green once it's been tested. Below the keyboard you'll see the name and code of the last key you pressed, plus how many times you've pressed it.
The code is more useful than it looks. It tells you which physical key your computer thinks you pressed, whatever language layout you use. On an AZERTY or German keyboard, the letters on screen might not match the caps on your keys, but the codes will still line up with the right position.
A key doesn't light up
First rule out software. Try the same key in a plain text editor like Notepad or TextEdit. If it's dead there too, the problem is the keyboard itself.
On a laptop, pay attention to the pattern. When a whole row or a block of keys fails at the same time, that usually points to the internal connector rather than the individual keys. That's a repair job. A single dead key on a mechanical keyboard is often just dust under the switch. On keyboards with hot-swap sockets, the switch may also have come loose slightly. Pull it out and push it back in firmly.
A key types twice
This is called chattering. You press once and the computer registers two presses. It's caused by a worn or faulty switch and it's common on mechanical keyboards after a few years of use.
You can check it here. Tap the suspicious key slowly, ten times, and watch the counter below the keyboard. If it says 12 or 13, you've found your problem. Blowing compressed air under the key sometimes helps. Otherwise the switch needs replacing, or you're looking at a new keyboard. Mouse buttons wear out the same way; the click speed test shows whether yours double clicks.
Keys don't register when pressed together
Hold down several keys at once and see how many light up. Cheaper keyboards often can't handle certain combinations of three or more keys. That's called ghosting, and it matters mostly in games where you hold movement keys while pressing something else. Keyboards advertised with "N-key rollover" (NKRO) should show every key you hold.
Keys doing something strange
Before you blame the hardware, check a few settings that catch people out all the time:
- F-keys change the volume or brightness instead. Your keyboard's Fn lock is on. On many laptops you toggle it with Fn + Esc.
- Keys respond slowly or not at all on Windows. Filter Keys may be turned on. Search for "Filter Keys" in the Start menu and switch it off.
- The numpad moves the cursor instead of typing numbers. Num Lock is off.
How this test works
Every press is recorded by the key's physical position, not by the character it types, so the test works the same on QWERTY, AZERTY and other layouts. The counter for a key goes up once per press; holding a key down doesn't add repeats, which is exactly why a key that registers twice stands out. Keys you hold at the same time stay highlighted, so you can see which combinations your keyboard can handle.
A few keys, such as Print Screen on Windows, only report being released, and the test counts those too. While the test is active it catches every key, including Tab, so to get out with the keyboard alone, press Esc three times quickly. Nothing you type is stored or sent anywhere.