What Does P0172 Mean on Your 2024 Subaru Forester?
A P0172 code on your 2024 Subaru Forester means the engine control module has detected that Bank 1 of your 2.5-liter Boxer flat-4 engine is running too rich. This engine produces 182 horsepower with port fuel injection. Subaru's horizontally opposed Boxer design has two banks — Bank 1 with cylinders 1 and 3 on one side. P0172 indicates that Bank 1 is receiving more fuel than the ECM can correct through fuel trim adjustments.
Symptoms You Might Notice
- Check engine light on
- Fuel economy below the rated 29 mpg combined
- Dark or black exhaust smoke
- Rough or unsteady idle
- Hesitation under acceleration
- Fuel smell from the exhaust
Common Causes of P0172 on the 2024 Forester
- Contaminated MAF Sensor — Even on a new vehicle, the MAF sensor can get dirty, especially if the Forester is used on unpaved roads. Incorrect airflow readings cause over-fueling.
- Defective Oxygen Sensor (Bank 1) — A factory-defective O2 sensor on the Bank 1 side provides wrong feedback, causing incorrect fuel calculations.
- Fuel Injector Issue on Bank 1 — A manufacturing defect in a port fuel injector can cause it to leak or deliver excess fuel.
- ECM Software Calibration — Software updates may be available for the 2024 model year to address fuel trim calibration.
How to Diagnose the Problem
- Visit your Subaru dealer — The 2024 Forester is under full warranty. Free diagnosis, TSB checks, and repairs.
- Check both banks — P0175 alongside P0172 suggests a system-wide cause like the MAF sensor.
- Monitor fuel trims — Bank 1 LTFT below -10% confirms the rich condition.
- Test the MAF sensor — At idle, the 2.5L Boxer should read about 3-8 g/s.
- Verify O2 sensor operation — Check Bank 1 upstream sensor for proper voltage switching.
Repair Options and Estimated Costs
- ECM software update — $0 under warranty
- MAF sensor cleaning or replacement — $10-$260 ($0 under warranty)
- Oxygen sensor replacement (Bank 1) — $170-$340 ($0 under warranty)
- Fuel injector replacement — $200-$450 ($0 under warranty)
- PCV valve check — $40-$100 ($0 under warranty)
All repairs are covered under Subaru's 3-year/36,000-mile basic warranty and 5-year/60,000-mile powertrain warranty.
Prevention Tips
- Replace the air filter more frequently if driving on dirt or gravel roads
- Use quality Top Tier gasoline
- Follow Subaru's recommended maintenance schedule
- Report check engine lights to your dealer promptly
- Keep the air intake system clean and properly sealed