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Engine

Misfires: spark plugs, coil packs and oil in the plug tubes

Iridium plugs are due every 60,000 miles on the turbo and need the airbox, battery and washer tank out to reach. Old plugs, a coil pack cracked by heat, or oil from the plug-tube seals cause misfire codes, and a flashing check engine light on a turbo car should not be driven hard.

OccasionalSeverity: ModerateApplicability: Direct

Frequency basis: Listed as a common generation-wide complaint (P0301–P0306, often at cold start) by Samarins, and tied to oil leaking into the spark plug wells in the XT oil-leak threads. Not a defect so much as a 60,000-mile service item that gets skipped because the plugs are hard to reach.

Symptoms

  • P0301–P0306 misfire codes, often first thing in the morning
  • Flashing check engine light under load (active misfire)
  • Rough idle and a stumble that sometimes clears when warm
  • Oil in the spark plug wells when the coils come out

Causes found

  • Worn plugs past the 60,000-mile interval, or plugs gapped outside 0.7–0.8 mm
  • Ignition coil failure (coil-on-plug, one per cylinder)
  • Oil from the spark plug tube seals and valve cover gasket fouling the plug and coil boot
  • Boost leaks causing lean misfires (see the boost-leak entry)

Check in this order

  • Read which cylinder is misfiring; swap the coil to another cylinder and see if the code follows
  • Pull the plug: oil on the outside means tube seals, a white or speckled insulator means detonation
  • Check plug gap and torque (21 N·m on dry threads)

What fixed it

  • New iridium plugs at the factory gap; most owners do the valve cover gaskets and tube seals at the same time because the access is shared
  • Replace the failed coil (22433AA540) rather than the whole set unless several are cracked
  • Fix boost or vacuum leaks before buying ignition parts

References on this site

OEM parts

  • 22401AA670Spark plug
  • 22433AA540Ignition coil assembly
  • 13293AA051Gasket, rocker (valve) cover

Maintenance guides

Sources

Forum threads are owner reports, not factory findings. Counts are from the threads and complaint records read for this page; read the sources before buying parts.

  1. 1.
    Subaru Outback 2005-2009: problems, pros and cons, reliability

    samarins.com · Article · misfire codes P0301–P0306, plugs first

  2. 2.
    05 XT Oil Leak - Pinpointing

    subaruoutback.org · Forum thread · plugs and valve cover gasket done together

  3. 3.
    Known Issues with 2005 Outback XT Limited???

    subaruoutback.org · Forum thread · coils and injectors "can go bad"