I've been chasing a mystery CEL the last few weeks, detecting a misfire at start up.
It randomly occurs during cold starts, but is not dependent upon ambient temperature and does not happen every time.
Sometimes you can feel the initial misfiring and it immediately smooths out, but usually there is no indication other than a CEL.
There is no misfire while driving, idle is smooth, power delivery is good, fuel economy has been normal.
It is usually associated with two codes.
ECM-3150: Misfire cylinder 5. Start up.
ECM-3100: Engine speed (RPM) sensor. Faulty signal.
Once, it also displayed
ECM-3130: Misfire cylinder 3. Start up.
I checked all spark plug gaps and they were well within spec.
I swapped cylinder 4 and 5 spark plugs and coil packs, the problem remains with cylinder 5.
I discovered software update 30668282, but that seems to trigger the RPM sensor faulty signal and the start up misfire warning for all cylinders.
Injectors? I would think a problem would be present more often than just start up if an injector was bad.
Air sensors? I feel like that wouldn't consistently pinpoint a misfire on cylinder 5.