What Does P0172 Mean on Your 2022 Subaru Outback?
When your 2022 Subaru Outback sets a P0172 code, the engine control module has detected that Bank 1 of your 2.5-liter Boxer flat-4 engine is running too rich. This horizontally opposed engine produces 182 horsepower and uses port fuel injection. Subaru's Boxer layout places cylinders horizontally in two banks — Bank 1 with cylinders 1 and 3 on one side. P0172 means that bank is receiving excess fuel beyond what the ECM can correct through fuel trim adjustments.
Symptoms You Might Notice
- Check engine light on
- Fuel economy below the rated 29 mpg combined
- Black or dark exhaust smoke
- Rough idle or slight vibration
- Hesitation or stumbling on acceleration
- Fuel smell from the exhaust
Common Causes of P0172 on the 2022 Outback
- Contaminated MAF Sensor — Outback owners who drive on gravel or dirt roads are especially susceptible. The MAF sensor gets dirty and underreports airflow, causing the ECM to inject too much fuel.
- Leaking Fuel Injectors on Bank 1 — Port injectors that drip or don't seal add unmetered fuel to the Bank 1 intake ports.
- Failing Bank 1 Oxygen Sensor — The upstream O2 sensor on the Bank 1 side may be degrading, providing inaccurate readings.
- Vacuum Leak or PCV Issue — A stuck PCV valve or cracked vacuum hose can affect the air-fuel balance.
How to Diagnose the Problem
- Check both banks — If P0175 is also present, the cause is system-wide. Bank 1 only means focus on that side of the boxer or the MAF sensor.
- Monitor fuel trims — Bank 1 LTFT below -10% confirms the rich condition.
- Clean the MAF sensor — At idle, the 2.5L Boxer should read about 3-8 g/s. Clean with MAF-specific spray.
- Check O2 sensor — Bank 1 upstream sensor should oscillate between 0.1V and 0.9V properly.
- Inspect the PCV valve — Ensure it moves freely and is not stuck closed.
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-$90 parts and labor
The 2022 Outback may still be under Subaru's 3-year/36,000-mile basic warranty. The powertrain warranty extends to 5 years/60,000 miles.
Prevention Tips
- Replace the air filter more frequently if driving on unpaved roads
- Use quality Top Tier gasoline
- Follow Subaru's maintenance schedule
- Address check engine lights promptly
- Keep the intake system clean and properly sealed