P0171 on Your 2020 Subaru Impreza
A P0171 code on your 2020 Subaru Impreza means the flat-four boxer engine is running lean on Bank 1. The 2020 Impreza uses a 2.0-liter boxer engine producing 152 horsepower. Bank 1 on the Subaru boxer is the passenger side.
The ECU detects the lean condition when the Bank 1 oxygen sensor readings cause fuel trim corrections to exceed their programmed limits.
Symptoms
- Check engine light on
- Rough idle
- Hesitation during acceleration
- Poor fuel economy
- Possible stalling
- Hissing from the engine area
Common Causes
- Vacuum Leaks - Cracked vacuum hoses, loose intake clamps, or intake manifold gasket leaks. The boxer layout has a wider intake spread with more connection points.
- MAF Sensor Issues - Contamination from dirt or oil vapors causes inaccurate readings.
- PCV Valve Failure - Creates an unmetered air leak into the intake.
- Fuel Delivery Problems - Aging fuel pump or dirty injectors on the Bank 1 side.
Diagnosis
- Scan all codes - Check for related codes.
- Monitor fuel trims - Compare Bank 1 and Bank 2 LTFT values.
- Smoke test - Find vacuum leaks in the boxer intake system.
- MAF and fuel tests - Verify specifications.
- PCV inspection - Check valve and hoses.
Repair Costs
- Vacuum hose repair - $50 to $200
- MAF sensor replacement - $120 to $270
- PCV valve replacement - $40 to $140
- Fuel pump replacement - $325 to $625
Diagnostic fee: $80 to $130.
Is It Safe to Drive?
Short commutes are fine for a couple of weeks. The Impreza's catalytic converters cost $600 to $1,100, so address the lean condition before it causes secondary damage.
Prevention Tips
- Follow Subaru's maintenance schedule
- Replace air filter regularly
- Use quality fuel
- Have vacuum hoses inspected during oil changes
- Check PCV system at major service intervals