P0172 Subaru Forester
System Too Rich (Bank 1) · Powertrain Code · Medium · Repair: Moderate
What is P0172?
The engine is running rich on bank 1 — too much fuel or not enough air. Common causes include leaky fuel injectors, stuck-open purge valve, or a dirty/faulty MAF sensor.
When the Subaru Forester triggers code P0172 (system too rich (bank 1)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Subaru Forester.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Subaru Forester.
Common Causes on Subaru Forester
- Contaminated or failed MAF sensor
- Leaking fuel injector
- High fuel pressure from faulty regulator
- PCV system issue pulling oil vapors into intake
- Clogged air filter reducing airflow to the boxer engine
Symptoms
- Check Engine Light on
- Fuel smell from exhaust
- Black soot on tailpipe
- Rough idle
- Fouled spark plugs
Notes for Subaru Forester
On FB25 engines with oil consumption issues, excess oil vapors through the PCV system can cause persistent rich codes.