Engine light on in my Subaru Impreza 2009

Asked by TRM_Prezzy Aug 19, 2019 at 12:24 PM about the 2009 Subaru Impreza 2.5i Hatchback

Question type: Maintenance & Repair

My engine light is on. I thought it was the spark
plugs, I changed them and my engine light went
off then, came back on a few hours later. I then
realized that my engine is knocking. I hear it when I
start my engine, it’s the loudest in first gear then as
I shift though the gears it gets lower. If my rpms
get too high my engine light starts to blink
meaning that whatever is malfunctioning is getting
more severe. I used an obd2 scanner and the
codes that are coming up are p0304 followed by
p0303 and p0301Can someone give me an
explanation as to what the problem might be

1 Answer


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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