What Does P0172 Mean on Your 2024 Subaru Crosstrek?
A P0172 code on your 2024 Subaru Crosstrek 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. The Boxer layout has two banks — Bank 1 contains cylinders 1 and 3 on one side. P0172 indicates excess fuel is being delivered to that bank beyond what the ECM can correct.
Symptoms You Might Notice
- Check engine light on
- Fuel economy below the rated 29 mpg combined
- Black or dark exhaust smoke
- Rough idle or vibration
- Hesitation during acceleration
- Fuel smell from the exhaust
Common Causes of P0172 on the 2024 Crosstrek
- Contaminated MAF Sensor — The Crosstrek is popular for outdoor adventures, and driving on dusty trails can contaminate the MAF sensor quickly, causing incorrect readings.
- Defective Bank 1 Oxygen Sensor — A factory-defective O2 sensor provides wrong feedback to the ECM about exhaust gas composition.
- Fuel Injector Issue on Bank 1 — A manufacturing defect in a port injector can cause it to leak or deliver excess fuel.
- ECM Software Update — Calibration updates may address fuel trim behavior on the 2024 Crosstrek.
How to Diagnose the Problem
- Visit your Subaru dealer — Under full warranty for free diagnosis, TSB checks, and repairs.
- Check both banks — P0175 alongside P0172 suggests a system-wide cause.
- Monitor fuel trims — Bank 1 LTFT below -10% confirms the rich condition.
- Test the MAF sensor — At idle, expect about 3-8 g/s for the 2.5L Boxer.
- Check O2 sensor — Verify Bank 1 upstream sensor switching properly.
Repair Options and Estimated Costs
- ECM software update — $0 under warranty
- MAF sensor cleaning or replacement — $10-$255 ($0 under warranty)
- Oxygen sensor replacement (Bank 1) — $165-$335 ($0 under warranty)
- Fuel injector replacement — $195-$440 ($0 under warranty)
- PCV valve replacement — $35-$90 ($0 under warranty)
All repairs are covered under Subaru's factory warranty.
Prevention Tips
- Replace the air filter more often if driving on unpaved roads or trails
- Use Top Tier gasoline
- Follow Subaru's maintenance schedule
- Report check engine lights to your dealer promptly
- Inspect the air intake system after off-road excursions