What Does P0172 Mean on Your 2022 Subaru Forester?
When your 2022 Subaru Forester triggers a P0172 code, the engine control module has detected that Bank 1 of your 2.5-liter Boxer flat-4 is running too rich. This engine produces 182 horsepower with port fuel injection. Subaru's Boxer layout has two banks — Bank 1 houses cylinders 1 and 3 on one side of the engine. P0172 means that side is getting excess fuel that the ECM cannot fully correct through fuel trim adjustments.
Symptoms You Might Notice
- Check engine light on
- Lower fuel economy than the rated 29 mpg combined
- Dark exhaust smoke
- Rough or vibrating idle
- Engine hesitation
- Fuel smell from the tailpipe
Common Causes of P0172 on the 2022 Forester
- Dirty MAF Sensor — The mass airflow sensor can accumulate contamination, especially on Foresters used off-road or on gravel. Incorrect readings cause over-fueling.
- Leaking Bank 1 Fuel Injectors — Port injectors that drip or don't seal add unmetered fuel to Bank 1 cylinders.
- Degrading Bank 1 Oxygen Sensor — After a few years, the upstream O2 sensor can slow down, giving inaccurate feedback.
- PCV Valve Malfunction — A stuck PCV valve pushes oil vapors into the intake, enriching the mixture.
How to Diagnose the Problem
- Check both banks — P0175 alongside P0172 points to a system-wide cause.
- Monitor fuel trims — Bank 1 LTFT below -10% confirms the rich condition.
- Clean the MAF sensor — At idle, expect about 3-8 g/s. Use MAF-specific spray.
- Test O2 sensor — Check Bank 1 upstream sensor for proper switching.
- Inspect Bank 1 spark plugs — Black, sooty plugs confirm rich running on that bank.
Repair Options and Estimated Costs
- MAF sensor cleaning — $10-$25 (easy DIY)
- MAF sensor replacement — $105-$255 parts and labor
- Oxygen sensor replacement (Bank 1) — $165-$330 parts and labor
- Fuel injector replacement — $190-$430 parts and labor
- PCV valve replacement — $35-$85 parts and labor
The 2022 Forester may still be under Subaru's basic or powertrain warranty depending on mileage.
Prevention Tips
- Replace the air filter frequently if driving off-road
- Use Top Tier gasoline
- Follow Subaru's maintenance schedule
- Address warning lights promptly
- Keep the intake system sealed and clean