Quality control must be going to hell, with plugs, ran into mis-numbered plugs so lay them all in a row first to make sure they all look alike, they all have to be regapped, lucky to find 2 out of eight that are right on. Also ran into leads between the metal base of the plug and the ceramic that causes blowby that can blow out the spark, so pull out my spark plug tester first. I know they are brand new name brand plugs and also feel you shouldn't have to do this. But the extra minutes spend can save hours and even the possibility of going insane trying to find a problem.
Always do a compression check with all the plugs out, and especially if the engine has a miss, if I find a low cylinder, pull out the cylinder leakage tester so I know the cause of the problem. But the latter is a PITA to use as have to hand turn the crank to the correct position. Also install a vacuum gauge first before taking stuff apart, with my altitude, expect to see 18"/Hg at idle, but depending on the engine, a vacuum leak can either cause a misfire or simply run the engine at a higher speed, so the tach also comes out.
Todays ignitions system really make me long for the days I only need my eyes to look at a set of points, have to swipe my wife's sewing needles to make contact for test points as they don't give you terminals anymore that were easy to access. And with DIS, distributorless ignition systems, have either 2,3,4, or even eight coils to test out instead of just one. Always put a drop of RTV on the pinholes I made or that cheap copper strands will corrode out. Have to pull out the scope, but have to admit I am guessing as they cannot give the courtesy to show waveforms, but at least know if the pulse is there or whether it's missing beats or not.
Fuel systems are equally maddening, with three instead of one fuel line to worry about, coupled with evaporative control, fuel pumps inside of the tank, pressure regulators with a very thin neoprene diaphragm that can rupture at any time, I don't like to think about that when driving, nor the millions of transistors in the PCM or BCM plus the countless electrical connections that can go bad. But do know I am in for a very long day if there is a problem somewhere.
And yes, I have returned electronic modules even though that big sign says you can't telling the counter guy, you sold me a piece of crap, already bench tested my old module and have the experience to make sure the new module is doing the job. Most of this stuff is put together by kids in China and doesn't even work out of the box.
Thinking about getting a horse. Good luck in finding your problem.