Quick answer: A Subaru Forester misfire on one cylinder usually stores P0301 through P0304, and on the boxer engine the common causes are a failed ignition coil, worn spark plugs, or oil fouling from leaking valve cover gaskets. Because the cylinders lie horizontally, oil leaks can run directly into the spark plug wells – a Subaru-specific failure mode.
The Forester’s horizontally opposed engine changes how misfires develop compared to an inline engine. This guide covers what to check first on this layout and why plug wells matter so much here.
Why the Forester Misfires
In a boxer engine the cylinders sit flat, so the spark plugs enter from the sides. When a valve cover gasket leaks – a common Subaru wear item – oil can seep down into the plug well and foul the plug or degrade the coil boot. The result is a persistent single-cylinder misfire that returns even after new plugs, until the leak itself is fixed.
For the full misfire explanation, see our main guide: P0300 – Random/Multiple Cylinder Misfire Explained.
Most Affected Forester Years
- 2009-2013 (EJ25): Valve cover gasket leaks and coil failures are frequent.
- 2014-2018 (FB25): Plug wear and coil issues; oil consumption on early FB engines.
- Any Forester past 90,000 miles: Spark plugs at or beyond service life.
Symptoms on the Forester
- Flashing check engine light during an active misfire.
- Rough idle and vibration, often worse when cold.
- Hesitation and reduced power on acceleration.
- Oil residue visible when the coil is removed – a strong Subaru clue.
Common Causes (Cheapest First)
- Worn spark plug – check the affected cylinder first.
- Failed ignition coil – swap with another cylinder to confirm.
- Oil in the spark plug well from a leaking valve cover gasket.
- Fuel injector fault or compression loss – less common, but investigate if the above are ruled out.
Check These Yourself First
- Note the exact cylinder from the code (P0301 = cylinder 1, and so on).
- Pull the coil and look for oil in the plug well – this is the Subaru-specific check.
- Swap the coil with a neighbouring cylinder; if the misfire follows, the coil is faulty.
Estimated Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Spark plugs (set of 4) | $30 – $90 | $100 – $220 |
| Ignition coil (one) | $40 – $120 | $60 – $130 |
| Valve cover gasket | $25 – $90 | $200 – $450 |
Note: 2026 US estimates. Boxer engines have restricted access on the sides, which is why labour on a valve cover gasket is higher than the part cost suggests.
Frequently Asked Questions
Why does oil get into the spark plug well on a Subaru?
The boxer layout places the plugs on the sides of the engine, below the valve covers. When a cover gasket leaks, gravity carries the oil directly into the plug tube, fouling the plug and coil boot.
My misfire came back after new plugs – why?
If oil is still leaking into the well, the new plug will foul as well. Fixing the valve cover gasket is what makes the repair permanent on this engine.
Is a flashing check engine light serious?
Yes. It indicates an active misfire that can damage the catalytic converter quickly. Reduce load and get it diagnosed promptly.
Related: P0301 cylinder 1 · P0300 full guide · P0420 catalyst