What Does P0174 Mean on Your 2020 Subaru Legacy?
The P0174 code on your 2020 Subaru Legacy means the engine control module has detected that Bank 2 of the 2.5L four-cylinder boxer engine is running too lean. The ECM is adding more fuel than normal to compensate for excess air, and it has run out of adjustment range.
Symptoms You Might Notice
- Illuminated check engine light
- Rough or unsteady idle
- Hesitation when accelerating
- Noticeable drop in fuel economy
- Occasional engine surging at idle
- Hissing sound from the engine bay indicating a vacuum leak
Common Causes of P0174 on the 2020 Legacy
The 2020 Legacy features the FB25 2.5L boxer engine. The horizontally-opposed cylinder layout can make certain repairs more challenging. Here are the top causes:
- Vacuum Leak — Cracked intake hoses, a leaking intake manifold gasket, or a stuck-open PCV valve are the leading causes. The boxer engine's horizontal orientation puts unique stress on these components.
- MAF Sensor Issues — The mass airflow sensor can get contaminated with oil from the air filter or crankcase ventilation system, causing inaccurate airflow readings.
- Fuel Pump Weakness — The 2020 Legacy uses an in-tank electric fuel pump that can weaken over time, causing low fuel pressure on Bank 2.
- Leaking Fuel Injectors — A clogged or partially stuck injector on Bank 2 reduces fuel delivery to those cylinders.
How to Diagnose the Problem
- Connect an OBD-II scanner and read all codes. Check if P0171 is also present.
- Review fuel trim data — long-term fuel trim above +10% on Bank 2 confirms the lean condition.
- Inspect all vacuum lines, intake boots, and the PCV valve visually.
- Perform a smoke test on the intake system to find hidden leaks.
- Test fuel pressure at the rail — the 2020 Legacy should read 40-58 PSI at idle.
- Clean the MAF sensor with proper cleaner and recheck data.
Repair Options and Estimated Costs
- Vacuum hose or gasket replacement: $100–$350. DIY-friendly once the leak is located.
- MAF sensor cleaning or replacement: $20–$250. Easy DIY repair.
- Fuel pump replacement: $400–$750 with parts and labor.
- Fuel injector service or replacement: $250–$550. Professional cleaning recommended.
Can You Still Drive?
You can drive your Legacy with a P0174 code for short trips, but do not delay repairs. A sustained lean condition raises combustion temperatures and can lead to catalytic converter damage, which costs $1,000 or more to replace on the Legacy.
Prevention Tips
- Replace the engine air filter every 15,000-30,000 miles per Subaru's schedule.
- Use quality Top Tier gasoline to keep the fuel system clean.
- Inspect vacuum hoses and intake connections during oil changes.
- Have the fuel system professionally cleaned every 30,000 miles.
- Do not ignore check engine lights — early diagnosis prevents costly repairs.