I have a Honda Civic DX, 1999. I was driving at ~20-30 mph, and heard a big noise and rattle. I open the hood and whoof comes the spark plug, flying in the air.
I was wondering why it blew off and if I should “just replace” it or go to a mechanic.
Maybe it was not tightened properly and it unscrewed itself. Very likely damaged the threads when it got loose enough to blow out. Take it to the mechanic.
Who changed the spark plugs previously?
Either too loose or too tight could have caused this with the latter likely leading to stripped threads.
Inspect the plug threads for aluminum and the cylinder head plug hole for damaged threads.
The spark plug was probably not properly torqued to specification. If you replace it, carefully check the threads, tighten it by hand first, and use a good torque wrench to make sure it is properly torqued in place.