A keyboard key is missing or repeating
Start with one key and one repeatable symptom. The fastest path is to prove whether the browser receives the event, then change one layer at a time.
Start with one key
Open the keyboard test, click the page once to confirm focus, then press the problem key ten deliberate times. Note whether the event never appears, appears only sometimes, or appears more than once per physical press. Those are different symptoms and should not be diagnosed the same way.
If the key never appears
First test an ordinary letter or number. Function-layer keys, media buttons and operating-system shortcuts can be intercepted before a webpage receives them. If an ordinary key is consistently absent, try another browser. Then reconnect the keyboard or change one port/connection method. If the same key remains absent on another computer, the evidence increasingly points toward the keyboard rather than the original browser.
If the key repeats unexpectedly
Separate a held-key repeat from duplicate presses. Browsers mark automatic key-repeat events differently from distinct physical presses, and operating-system repeat delay/rate settings can change how a held key behaves. Test with short taps first. If one tap repeatedly produces multiple distinct events across browsers or machines, use the keyboard maker's diagnostic or support process before deciding the switch is faulty.
If combinations fail but single keys work
That is a different problem. Open the rollover test and reproduce the exact combination. Keyboard matrix limitations, firmware behavior and reserved shortcuts can all affect combinations even when every key works alone. The ghosting and rollover guide explains that boundary in detail.
Wireless and laptop checks
For wireless keyboards, compare a charged battery and a stable connection before changing software settings. For laptops, remember that Fn layers and vendor hotkeys may be handled outside normal browser keyboard events. An external USB keyboard can be a useful comparison because it changes the input device without changing the computer.
When the browser test is enough
If the key works normally in BenchRush and in another browser, the physical switch may still have an intermittent fault, but the current browser session did not reproduce it. If the failure follows the same physical key across browsers, ports and computers, you have stronger evidence to take to the manufacturer. BenchRush does not issue a hardware pass/fail certificate.
Technical reference
BenchRush reads standard browser keyboard events. MDN documents the browser-visible key value in KeyboardEvent.key. That API describes what reaches the page, not the electrical state of a keyboard switch.