What P0171 Means on a 2022 Subaru Forester
A P0171 trouble code on your 2022 Subaru Forester indicates the engine is running lean on Bank 1. The 2022 Forester uses a 2.5-liter flat-four boxer engine producing 182 horsepower. Bank 1 on the Subaru boxer is the passenger side of the engine.
When the upstream O2 sensor on Bank 1 detects a consistently lean mixture and fuel trim corrections reach their maximum, the ECU stores P0171.
Symptoms
- Check engine light on
- Rough or shaking idle
- Hesitation when pressing the gas
- Reduced fuel economy
- Possible stalling
- Hissing noise from the engine
Common Causes
- Vacuum Leaks - The boxer layout creates more intake plumbing connections. Cracked hoses, loose clamps, and gasket leaks are common.
- MAF Sensor Contamination - Dirt and oil vapors coat the MAF sensor, causing incorrect airflow readings.
- PCV Valve Failure - A stuck PCV valve creates an air leak into the intake.
- Fuel System Issues - Weak fuel pump or dirty injectors on Bank 1.
Diagnosis
- Scan all codes - Check for related codes.
- Monitor fuel trims - Compare Bank 1 and Bank 2 LTFT values.
- Smoke test - Essential for the boxer engine layout.
- MAF sensor test - Compare to boxer engine specifications.
- Fuel pressure test - Verify adequate delivery.
Repair Costs
- Vacuum hose repair - $60 to $250
- MAF sensor replacement - $140 to $300
- PCV valve replacement - $45 to $160
- Fuel pump replacement - $400 to $725
Diagnostic fee: $90 to $150.
Can You Drive?
Short drives are safe for a week or two. Avoid extended trips until repaired. The Forester's catalytic converters cost $700 to $1,300 and can be damaged by prolonged lean operation.
Prevention Tips
- Follow Subaru's maintenance schedule
- Replace the air filter regularly
- Use quality gasoline
- Have vacuum hoses checked during oil changes
- Inspect the PCV system at major service intervals