Head gasket failure in your 2016 Subaru Outback at 100,000 miles continues the long history of head gasket issues in Subaru's boxer engines. While the newer FB engine in the 2016 is more reliable than the notorious EJ engines, head gasket problems still occur.
Subaru Head Gasket Background
Subaru's horizontally opposed "boxer" engines have cylinders extending horizontally from the crankcase. This design places stress on head gasket sealing surfaces. Earlier EJ engines (pre-2011) had widespread gasket failures; the FB engine improved but didn't eliminate the issue entirely.
Types of Head Gasket Failure
External coolant leak - coolant weeps from between head and block, often visible on the engine sides. Internal coolant leak - coolant enters cylinders or oil passages without external evidence. Combustion gas leak - compression gases enter the cooling system, causing overheating and bubbling in the reservoir.
Code P0171 Connection
A lean condition code with head gasket issues suggests combustion gases in the cooling system affecting engine management. It can also indicate vacuum leaks from gasket breach. If accompanied by coolant loss or overheating, suspect head gasket involvement.
Repair Reality
Head gasket replacement on a boxer engine is a major repair - the engine design makes access difficult. Expect $1,500-2,500 for the repair. Both head gaskets should be replaced together. At 100,000 miles, also inspect timing components and water pump while the engine is apart.