Sure. You probably disconnected the battery when changing the plugs, thus requiring
three starts to reflag the CEL. This is normal.
Many 2.5i of that gen will "knock" on cold start, but it's really piston slap, and reduces
with warmup and rpm. Annoying, but not problematic. BUT, if, after full warmup, you DO
hear actual rod knock upon acceleration then you may have a worm bottom end from
prior oil starvation, and you're cruising on an iffy plateau.
The PO30x codes are misfires, the last digit indicating the cylinder involved.
These will occur often if the ignition wires are draped too close to the intake manifold
during damp weather. Twist the wires on the coil-pack to run at least 1/4" away from
metal, as well checking their connections for cleanliness.
If wires and plugs are absolutely secure, then you may have a cracked coil-pack. These
are cheap and easy to swap out, as you can see.
Then, if one PO30x recurs after you're assured that it's NOT ignition, you can try
swapping the fuel injectors on that head to see if the CEL goes to the other cylinder,
indicating a clogged or intermittent injector. But if the PO30x stays the same, then you
have an internal valve problem, and should probably tear down both heads, lap the
valves, and replace head gaskets while in there. But this is $1800 or so.
Simple stuff first, eh? Ern TSG/B
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