Head gasket failure in your 2010 Subaru Outback's EJ25 engine at 200,000 miles is the culmination of Subaru's most well-known reliability issue. The EJ25 boxer engine has experienced head gasket problems since the late 1990s, and by 2010, Subaru had made improvements but not eliminated the issue entirely.
The EJ25 Head Gasket Legacy
Subaru's horizontally-opposed engine design places unique stress on head gaskets. The 2.5L EJ engine has had gasket failures affecting multiple generations of Outback, Legacy, Forester, and Impreza. At 200,000 miles, you may be on your second head gasket if the original was replaced earlier.
Types of EJ25 Head Gasket Failure
External coolant leak - coolant weeps from between head and block, visible on the engine sides. Internal combustion leak - compression enters the cooling system, causing overheating and bubbles in the reservoir. Internal coolant-oil leak - coolant mixes with oil, visible as milky substance on dipstick.
Code P0171 Connection
This lean code can result from vacuum leaks at a failing head gasket or from combustion issues affecting fuel calculations. It's not specific to head gaskets but often accompanies the problem.
Repair at 200K Miles
Head gasket replacement is a significant repair - $1,800-2,500 typically. At 200,000 miles, also consider timing belt/water pump, valve cover gaskets, and other maintenance while the engine is apart. If the engine has other issues, an engine swap may be more practical than repair.