Understanding P0171 on the 2022 Subaru Impreza
If your 2022 Subaru Impreza has triggered a P0171 code, the boxer engine is running lean on Bank 1. The 2022 Impreza uses a 2.0-liter flat-four boxer engine producing 152 horsepower. Bank 1 on Subaru's boxer layout is the passenger side of the engine.
When the upstream O2 sensor on Bank 1 detects a persistent lean condition and fuel trim adjustments exceed their limits, P0171 is stored and the check engine light illuminates.
Symptoms
- Check engine light on
- Rough idle
- Hesitation when accelerating
- Lower fuel economy
- Possible stalling at stops
- Hissing from under the hood
Common Causes
- Vacuum Leaks - The boxer engine's wide intake layout has multiple connection points. Cracked hoses and loose clamps are the primary cause.
- MAF Sensor Contamination - Dust and oil vapors degrade sensor accuracy.
- PCV System Issues - Failed PCV valve or cracked breather hose.
- Fuel System Problems - Weak pump or dirty injectors.
Diagnosis
- Full code scan - Check for companion codes.
- Fuel trim monitoring - Compare Bank 1 and Bank 2 LTFT.
- Smoke test - Locate vacuum leaks.
- MAF and fuel pressure tests - Verify specifications.
- PCV check - Inspect valve and hoses.
Repair Costs
- Vacuum hose repair - $50 to $225
- MAF sensor replacement - $125 to $275
- PCV valve replacement - $40 to $150
- Fuel pump replacement - $350 to $650
Diagnostic fee: $80 to $140.
Can You Drive?
Short trips are OK for two weeks. Get it fixed before the catalytic converter is damaged ($600-$1,100 replacement).
Prevention Tips
- Follow Subaru's maintenance schedule
- Replace air filter regularly
- Use quality gasoline
- Inspect vacuum hoses at oil changes
- Check PCV system at major services