What Does This Error Mean?

Subaru EJ25 SOHC engines (non-turbocharged Outback, Forester, Legacy, and Impreza from approximately 1999–2012) have a widely documented head gasket weakness. Typical failure: external coolant seep that slowly worsens over time — white or orange residue visible on the engine block. Early detection avoids a far more expensive internal failure.

Affected Models

Subaru Outback (EJ25 SOHC)
Subaru Forester (EJ25 SOHC)
Subaru Legacy (EJ25 SOHC)
Subaru Impreza (EJ25 SOHC)
Subaru Baja (EJ25 SOHC)

Common Causes

Head gasket design and material issue specific to the EJ25 SOHC engine — Subaru revised the gasket multiple times but the issue persists on original units
Overheating event accelerating gasket failure
Age and heat cycling degrading the gasket material over time
External coolant seep (most common) — coolant leaks out between the head and block rather than internally
Internal failure (less common but more serious) — combustion gases enter the coolant or coolant enters the cylinders

How to Fix It

Look for dried coolant residue on the engine block.

The most common EJ25 head gasket failure is an external coolant seep — coolant slowly weeps out from the joint between the cylinder head and engine block. Look for dried white, orange, or brown staining on the outside of the engine block in the area where the heads bolt on (on each side of the Boxer engine). This residue is dried coolant and is a strong indicator of a seeping head gasket.
Monitor coolant level regularly.

A seeping head gasket causes gradual coolant loss — you may notice the overflow tank level dropping between services. Check the coolant level every 1,000 km — if it is consistently dropping with no visible external puddle under the car, the coolant is likely being lost through a head gasket seep. Do not simply keep topping up the coolant without finding the source.
Watch for white exhaust smoke.

If coolant enters the combustion chamber (internal head gasket failure), the engine burns coolant and produces thick white or grey smoke from the exhaust — different from the normal water vapour on cold mornings. This smoke has a slightly sweet smell and does not disappear after the engine warms up. Internal failure is more serious than an external seep — get the vehicle to a Subaru specialist immediately.
Check the oil for coolant contamination.

Pull the oil dipstick and look for a milky or frothy appearance — this indicates coolant is mixing with the engine oil. Also check the oil filler cap: a creamy white residue on the underside is a sign of coolant contamination. Oil and coolant mixing is a sign of internal head gasket failure and causes severe bearing damage if not addressed promptly.
Have a combustion gas test performed.

A combustion gas test (block test) uses a chemical indicator placed over the coolant reservoir to detect combustion gases leaking into the cooling system — a definitive test for internal head gasket failure. This test takes about 10 minutes and gives a clear positive or negative result. A Subaru-experienced mechanic or specialist can perform this test — it is also available as a DIY kit from auto parts stores.

When to Call a Professional

Head gasket replacement on a Subaru EJ25 engine is a significant repair — the engine must be partially disassembled to reach the head gaskets. Get quotes from at least two Subaru specialists. The repair is worthwhile on vehicles with otherwise low mileage and good condition.