Tools for testing and replacing spark plugs
When replacing a spark plug, you need special tools to do the job easily. Spark plugs are usually screwed in deeply, which means that an ordinary hex cap will not give you enough grip. A spark plug cap offers the solution because these are usually longer and have rubber in the cap which keeps the spark plug firmly in place when installing or removing the spark plug. There are different sizes of spark plugs hence there are also different spark plug wrenches from 8 to 21 mm. Below you will also find various tools to test a spark plug and to repair the threads.
In short, here you will find all the tools to independently replace and test a spark plug.

BGS TECHNIC Spark plug wrench with T-handle, 21mm

LASER 3/8" (10mm) Spark Plug Wrench Set, T-Handle, 16,18,21mm

GUNSON Colortune, Spark Plug Tester 10mm

GUNSON Colortune, Spark Plug Tester 14mm| GUNSON G4074

GUNSON Colortune Adapter 10mm For Spark Plug Tester | GUNSON G4055a

KS-TOOLS 3/8 "spark Plug Cap For Ford Ecoboost Engines,14 And 16mm 2-piece

FACOM 1/2" (12,5mm) Bougiedop 21mm, 250mm Lang

GUNSON Colortune Adapter 12mm Voor Bougie Tester | GUNSON G4055b

BGS TECHNIC 3/8"(10mm) Twaalfkant Bougiedopsleutel 14mm

BGS TECHNIC 3/8"(10mm) Twaalfkant Bougiedopsleutel 16mm

BGS TECHNIC 3/8"(10mm) Twaalfkant Bougiedopsleutel 18mm