What P0171 Means on Your 2020 Subaru Legacy
A P0171 code on your 2020 Subaru Legacy means the engine is running lean on Bank 1. The 2020 Legacy features a 2.5-liter flat-four boxer engine producing 182 horsepower or an available 2.4-liter turbocharged boxer with 260 horsepower. Bank 1 on the boxer layout is the passenger side.
The ECU monitors the air-fuel ratio through the upstream O2 sensor on Bank 1. When fuel trim corrections exceed their limits, P0171 is stored.
Symptoms
- Check engine light on
- Rough idle
- Hesitation during acceleration
- Worse fuel economy
- Possible stalling
- Hissing from the engine
Common Causes
- Vacuum Leaks - The boxer engine's wide intake system has multiple potential leak points at hoses, clamps, and gaskets.
- MAF Sensor Contamination - Dirt and oil degrade MAF accuracy.
- PCV Valve Failure - Creates unmetered air leak.
- Fuel Delivery Issues - Weak fuel pump or dirty injectors.
Diagnosis
- Scan all codes - Check for companion codes.
- Fuel trim analysis - Compare banks.
- Smoke test - Find vacuum leaks in the boxer intake.
- MAF and fuel pressure tests
- PCV system inspection
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 trips are safe for two weeks. Avoid extended highway driving. The Legacy's catalytic converters cost $700-$1,300 and can be damaged by prolonged lean operation.
Prevention Tips
- Follow Subaru's maintenance schedule
- Replace air filter regularly
- Use quality gasoline
- Have vacuum hoses inspected at services
- Check PCV system at major intervals