The P0172 code in your 2013 Subaru Forester indicates the fuel system is running too rich, and when combined with excessive oil consumption, this suggests oil is entering the combustion chamber and being burned. The 2013 Forester's 2.5L FB25 engine has documented oil consumption issues that Subaru addressed through a class action settlement.
Understanding Oil Consumption and Rich Codes
When oil enters the combustion chamber and burns, it affects the oxygen sensor readings. The ECM may interpret this as a rich condition and set P0172 as it tries to lean out the mixture. On the FB25 engine, oil consumption exceeding 1 quart per 1,200 miles was common enough to prompt legal action and an extended warranty program from Subaru.
Causes of Excessive Oil Consumption
The primary cause on the FB25 is piston ring design that allows oil to pass into the combustion chamber. The rings may not seat properly against the cylinder walls under certain operating conditions. A faulty PCV valve or system can increase crankcase pressure and push oil past seals and rings. Worn valve seals allow oil to seep down valve stems into cylinders.
Diagnostic Process
Monitor oil consumption by checking level every 1,000 miles and documenting how much you add. Look for blue smoke from the exhaust, especially on startup or during deceleration. Check PCV valve operation—a stuck valve can worsen oil consumption. Perform a compression test to evaluate piston ring seal. Review if your vehicle qualifies for Subaru's oil consumption warranty extension or settlement.
Repair Options
Check eligibility for Subaru's oil consumption program—if qualified, piston and ring replacement may be covered. If not covered, evaluate whether repair cost is justified for your vehicle. Some owners simply monitor and maintain oil level rather than undertake expensive repairs. Ensure PCV system is functioning properly and use Subaru-recommended oil weight. Consider switching to high-mileage oil formulations designed to condition seals.