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.

CUSTOR 3/8" (10mm) Spark Plug Socket Set, 10-Piece

LASER 3/8" (10mm) Spark plug cap 16mm

CUSTOR 1/2" (12.5mm) Spark plug cap 16mm

CUSTOR 1/2" (12.5mm) Spark plug cap 21mm

DELTACH Repair Kit Spark Plug Thread, 17-Piece

KS-TOOLS 3/8" (10mm) Spark Plug Wrench Set With Knee Joint, 6-Piece

BGS TECHNIC Spark plug wire repair kit M14x1.25

FACOM 1/2" (12.5mm) Spark Plug Cap 16mm

FACOM 1/2" (12.5mm) Spark Plug Cap 18mm

FACOM 3/8" (10mm) Spark Plug Cap14mm

FACOM 3/8" (10mm) Spark Plug Cap 16mm, Long

FACOM Pipe Wrench for Spark Plug From 16mm

FACOM Pipe Wrench for Spark Plug From 16mm

FACOM Pipe Wrench for Spark Plug From 21mm

FACOM Pipe Wrench for Spark Plug From 21mm

LASER 3/8" (10mm) Spark Plug Cap With Knee Joint 14mm

FACOM 3/8" (10mm) Spark Plug Wrench With Knee Joint 8mm

LASER Glow Plug Wrench Set, 18-Piece

FACOM Flexible Assembly Key for Spark Plug

SINATEC Brush Set, 3-Piece

SW-STAHL Spark Plug Wrench With T-Handle, 16mm

ELORA Spark Plug Brush With Brass Wire

XPTOOLS Glow plug removal kit, 16-piece

DRAPER Spark Plug Ignition Tester