High oil consumption in your 2018 Subaru Legacy accompanied by a P0171 lean code presents an interconnected problem. Oil burning affects combustion and can contaminate sensors, while the underlying causes of consumption may also contribute to the lean running condition.
Connection Between Oil and Lean Codes
Oil consumption and lean codes can connect through several mechanisms: oil fouling the MAF sensor causes incorrect air measurement, PCV system issues causing both oil consumption and vacuum leaks, and oil coating oxygen sensors affecting fuel trim accuracy.
Oil Consumption in the FB25 Engine
The Legacy's 2.5L FB25 boxer engine has documented oil consumption tendencies affecting some owners more than others. Piston ring design, driving patterns, and oil quality all influence consumption rates. Subaru considers up to 1 quart per 1,200 miles acceptable, though most owners expect less.
P0171 Lean Causes
Beyond oil-related causes, standard lean condition sources include vacuum leaks at the intake manifold or PCV system, dirty or contaminated MAF sensor, weak fuel pump or restricted filter, and air filter restrictions affecting calculated air mass.
Diagnostic Priority
Determine if the lean condition and oil consumption are related or separate issues. A contaminated MAF from oil blow-by connects them directly. Unrelated causes require separate repairs.