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The SOHC EJ25's defining flaw is an external head-gasket leak: the gasket seeps oil and/or coolant at the head-to-block seam, classically once the car is well used (the corpus median on these complaints is ~86k miles, and it is a near-certainty by ~100k+ on an un-serviced example). It is the best-known Subaru issue of this era -- 237 federal complaints across the SOHC EJ25 models (Outback, Forester, Legacy, Impreza, MY2000-2011) name a failed head gasket, led by the Outback (120). It is usually a slow external seep rather than a sudden failure, but a correct repair -- heads off, resurfaced, timing components refreshed -- runs well into four figures, so it is a price-negotiation item more than a walk-away.

Check — Look for oily or crusty coolant residue where the heads meet the block, low or recently topped-off coolant, an oil sheen in the coolant or a milky film under the oil cap, and any overheating history. Ask for records of a prior head-gasket job and whether the timing belt, tensioner, and water pump were done with it.

The reason the head gasket matters is where a neglected leak leads: coolant loss and overheating. Among these SOHC EJ25 engine complaints, 51 describe overheating and 32 tie a head gasket directly to an overheating event. The boxer's aluminum heads warp if the engine is driven hot, which turns a gasket job into a cylinder-head or whole-engine job. Owners frequently report the seep was slow and easy to ignore for months before the temperature gauge finally moved.

Check — Watch the temperature gauge through a full warm-up and a hard pull; confirm the coolant is full and clean (not oily or rusty), the radiator fans cycle, and there is no sweet coolant smell or white exhaust. Treat any history of overheating on one of these as a heads-came-off-or-should-have question.