Spark, at the right time
Fuel, in the right mix with air
Compression, above 100psi
Those are the big 3 a gas engine needs to ignite a cylinder.
Compression can't really be intermittent, so it is often best to check it then it can be taken off the table.
2.3l's have been know to have rockers wear down so valves don't open all the way, also lean running can cause "tulipped" valve, so low compression with either.
As RPMs increase compression does too, so cylinder can start firing at that time.
Spark can be test with, old timing light or spark tester.
Fuel can be checked by disabling spark, unplug coils, and cranking engine over, then pull spark plug and see if it is fuel on it.
Can do this test when testing compression as well, check and wipe off end of tester after testing each cylinder.
Compression test must be done with 1 spark plug removed from each cylinder, so crank shaft speed is faster.
FYI on the 2.3l or 2.5l, only the exhaust side spark plugs/coil are used when starting, intake side don't start until engine is running, i.e. RPMs above 600