Test it the same way, then test it again wired
Run through every key in the tester above as you would with any keyboard. Then, if the keyboard has a cable option, connect it and run the same test. A fault that disappears on the cable was never a key.
If the keyboard is Bluetooth only, move it within a metre of the computer and retest before concluding anything.
What wireless problems look like
- Missed keys that are different every time. A hardware fault kills the same key every time; a radio problem is random.
- Doubled characters, caused by a repeated packet or a lost release rather than switch chatter.
- A burst of keys arriving late together after a pause, which is a link that stalled and caught up.
- The first keystroke after a pause being swallowed, which is the keyboard waking from a power-saving sleep.
Things that reliably help
- Move the USB receiver to a front port or an extension cable, away from the metal case and away from a USB 3 port, which is a known source of interference in the 2.4GHz band.
- Replace the batteries. Wireless keyboards behave erratically long before they stop entirely, and a weak battery produces exactly the dropped-key symptoms above.
- Remove and re-pair the Bluetooth connection. A stale pairing causes lag and dropouts that look like failing hardware.
- Move other 2.4GHz devices away, including wireless mice with their own receivers, and check whether a nearby router is on a congested channel.
When it really is the keyboard
If the same key fails wired, fails at one metre, fails on a second computer and fails with fresh batteries, it is the switch. At that point it is an ordinary keyboard repair and the pages on chatter and stuck keys apply.
Bear in mind that many wireless keyboards are membrane designs where individual switches are not replaceable, so the repair may not be economic.
Bluetooth and multi-device keyboards
Keyboards that pair with several devices and switch between them with a dedicated key introduce a failure mode of their own: the keyboard is connected to a different machine than the one you are looking at. Everything appears dead, and nothing is wrong.
Press the device-switch key for the machine in front of you and watch the tester. If keys start reporting immediately, that was the whole problem. It is worth ruling out first, because it costs three seconds and explains a surprising share of dead wireless keyboards.
A keyboard that was paired to a machine that has since been reinstalled or renamed can also hold a stale pairing that connects but never delivers keys. Removing the pairing on both sides and starting again clears it.
Latency is not a fault
The held time in the readout measures how long you held a key, not how long the key took to arrive, so the tester cannot measure wireless lag directly. A modern 2.4GHz keyboard adds a few milliseconds, which is below the threshold anyone notices while typing.
If a wireless keyboard genuinely feels laggy, the usual causes are a receiver behind a metal case, a congested 2.4GHz band, or a power-saving mode that puts the link to sleep between bursts of typing. The first keystroke after a pause being slow, with everything after it fine, is the signature of that last one.