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P0303 - next step?

Hi. 2006 Forester 160K miles. All of a sudden, very rough idle, good at higher speed. Blinking CC; CEL on or blink. Initial diag: P0303, compression <<100 on #3. I have to use a new dealer so no track record for trust. Does the low compression rule out simple problem like wire, plug, coil, etc.? What would you check next? New to Subaru; Thanks!


  • ateixeiraateixeira Posts: 72,587
    Gotta be a head gasket I'd think...
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,507
    Low compression could be many things, including a sticking valve, timing, rings, head gasket, etc. It probably isn't a "simple problem" like the wire or plug. More testing is needed to pinpoint the problem, starting with a leakdown test.
  • Thanks. I'm curious why it's better at driving speeds?
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,507
    Interesting. I glazed over that part as I read the low compression information. I had a '96 Outback that suddenly was horribly rough during idle, but (like yours) was fine at higher speeds. The problem with that one was a knock sensor. But, the knock sensor failed, which did post as a trouble code (knock sensor out of spec). It ran especially poor when the engine was cold.
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