Which layout to pick
Most laptops are closest to the tenkeyless layout: full function row, arrows, no number pad. Larger machines with a number pad match the full size layout instead. Pick whichever matches what is in front of you; keys the layout does not draw are still recorded and named in the readout.
The exact position of Home, End, Page Up and Page Down varies more on laptops than on any other keyboard, so expect the drawn board to differ slightly from yours in that region.
The Fn layer
Laptops put a second set of functions on the top row, and often on the number pad overlay and the arrow keys as well. Which set is active by default is set in firmware, usually described as Action Keys Mode, Hotkey Mode or Function Key Behaviour in the BIOS.
Holding Fn will not show anything in the tester, because Fn is resolved inside the keyboard. Holding Fn and pressing a top-row key will show either the function key or nothing, and that tells you which mode the machine is in.
What laptop keyboards fail from
- Debris under low-profile keys, which have very little travel and very little tolerance for it.
- Liquid, which on a laptop reaches the board below rather than draining out of a case.
- The ribbon cable connector working loose, which usually kills a contiguous block of keys rather than one.
- Wear on the scissor clips under heavily used keys, leaving a cap that feels loose and registers unreliably.
Before assuming hardware
Plug in a USB keyboard and test that. If the external keyboard has the same dead keys, the problem is software or the operating system layout rather than the laptop keyboard.
Check that no remapping tool is running, and that Sticky Keys and Filter Keys are off. On a work machine, a management tool can also remap keys without any visible sign.
Test in the BIOS if you can reach it. Keys that work there and not in the operating system are a software problem, which is much better news than the alternative.
On most laptops a single dead key means replacing the keyboard assembly. An external keyboard is a perfectly reasonable fix for a machine that lives on a desk.
Buying a used laptop
A full key pass is the cheapest five minutes you can spend before handing over money for a second-hand machine. Keyboards are the part most likely to have been abused, the part sellers are least likely to mention, and on most models the most expensive small component to replace.
Work across every key in order and watch the untested count reach zero. Then hold a key down for a couple of seconds to check it is not chattering, and press the shift, control and alt keys on both sides, since left and right report separately and one of a pair failing is common.
Do the same for the trackpad buttons and the hinge while you are there. Neither is testable here, but both are in the same category of expensive to fix and easy to overlook.
Manufacturer diagnostics
Most laptop makers ship a hardware test that runs outside the operating system, usually reachable from a key held at power on. Those tests can distinguish a failed keyboard assembly from a controller problem in a way a browser cannot, and the result carries weight with a warranty claim.
Use this tester first, because it takes seconds and answers most questions. Use the manufacturer tool when you need evidence rather than an answer, or when the operating system itself is suspect.