P0171 on Your 2024 Subaru Forester
A P0171 code on your 2024 Subaru Forester means the engine is running lean on Bank 1. The 2024 Forester uses a 2.5-liter flat-four boxer engine producing 182 horsepower. On Subaru's boxer layout, Bank 1 is the passenger side of the engine.
The ECU monitors combustion through the upstream oxygen sensor and adjusts fuel delivery via fuel trims. When corrections exceed their limits on the lean side, P0171 is triggered.
Symptoms
- Check engine light illuminated
- Rough idle
- Hesitation under acceleration
- Decreased fuel economy
- Possible stalling
- Hissing from under the hood
Common Causes
- Vacuum Leaks - The boxer engine layout has a wide intake manifold with multiple connections. Loose clamps, cracked hoses, or leaking gaskets are common.
- MAF Sensor Contamination - Dirt and moisture in the airbox can contaminate the MAF sensor.
- PCV System Issues - Failed PCV valve or cracked breather hose.
- Software Calibration - Newer Subaru models may benefit from ECU software updates.
Diagnosis
- Full code scan - Check for companion codes.
- Fuel trim analysis - Monitor Bank 1 vs Bank 2 trims.
- Smoke test - Best method for finding leaks on the boxer engine.
- MAF sensor test - Compare to boxer engine specs.
- Check for TSBs - Look for applicable technical service bulletins.
Repair Costs
- Vacuum hose or gasket repair - $75 to $275
- MAF sensor replacement - $150 to $325
- PCV valve replacement - $50 to $175
- ECU software update - $0 to $150 (often free at dealer)
Diagnostic fee: $100 to $150.
Warranty
Your 2024 Forester is under Subaru's 3-year/36,000-mile basic warranty and 5-year/60,000-mile powertrain warranty. Take it to a Subaru dealer for covered repair.
Prevention Tips
- Follow Subaru's maintenance schedule
- Replace air filters at recommended intervals
- Use quality fuel
- Have vacuum connections inspected during services
- Allow the dealer to install software updates