Start with the key itself
Press the suspect key a few times, then press a couple of nearby keys for comparison. Make sure the keyboard model and layout shown by the tester broadly match the keyboard in front of you.
If the key is observed, that press reached the browser. If it is not observed, keep going: the cause could be an application or system setting, a connection problem, a browser limitation or a physical fault.
Try the same key somewhere else
Test the key in another ordinary application where the result is easy to see, such as a text editor, and then try another browser if you can. This helps separate a problem that is specific to one webpage or application from a problem that follows the keyboard more widely.
If the key works in the tester but not in one application, check that application and your operating-system input settings before assuming the keyboard is faulty. Keyboard layout, key-remapping and accessibility settings can all change how a key behaves.
Check the keyboard connection
For an external wired keyboard, disconnect and reconnect it firmly. If practical, try a different USB port and connect directly to the computer rather than through a hub while you are testing.
For a wireless keyboard, check that it is powered, charged or has fresh batteries, is in range and is still connected. Turning the keyboard off and on or reconnecting its Bluetooth connection can resolve a connection problem.
Compare with another device or keyboard
If you can, try the keyboard on another compatible computer. You can also try a different keyboard on the original computer. The pattern is often more useful than any single test result.
If the same ordinary key is missing in different applications and on another computer, while a different keyboard works normally on the original computer, that points more strongly to the keyboard itself. It still does not prove the exact hardware fault.
Look for a physical problem
If the key feels different, sticks, has been exposed to dirt or liquid, or only responds when pressed unusually hard, a physical problem is more likely. Before cleaning or removing anything, follow the keyboard or computer manufacturer’s instructions for your exact model.
Avoid forcing keycaps off a laptop or opening a keyboard just to diagnose one key. If safe software, connection and manufacturer-approved cleaning checks do not help, use the manufacturer’s repair guidance or a qualified repair service.
Some keys need a different interpretation
Fn and some media, system and shortcut keys are special cases. Your keyboard, browser or operating system can handle them before a normal webpage can see the press, so a key that does not appear here is not automatically broken.
If several keys fail together
A group of keys that only stops registering when certain keys are held together is a different problem from one consistently unresponsive key. The browser can show the simultaneous key presses it receives, but missing presses alone do not prove a specific ghosting or rollover fault.
Troubleshoot several keys that aren't registering together →
When a hardware fault becomes more likely
A physical keyboard problem is more likely when an ordinary key repeatedly fails across different applications or browsers and the same problem follows the keyboard to another compatible device. Check connection and relevant software settings first.
That is the point to check the manufacturer’s support or repair options. Do not replace a keyboard solely because one press was not observed by this website.