What Does P0174 Mean on Your 2023 Subaru Ascent?
The P0174 diagnostic trouble code on your 2023 Subaru Ascent indicates that Bank 2 of the 2.4L turbocharged boxer engine is running too lean. The powertrain control module has been adding extra fuel to correct the mixture, but it has maxed out its adjustment capability.
Symptoms You Might Notice
- Check engine light on your dashboard
- Rough or unstable idle
- Noticeably reduced power, especially under boost
- Lower fuel economy than normal
- Hesitation or stumbling when accelerating
- Possible whistling or hissing from the engine or turbo piping
Common Causes of P0174 on the 2023 Ascent
The 2023 Ascent uses the FA24 2.4L direct-injection turbo engine. The forced induction system adds several potential failure points:
- Boost Leak — Loose clamps, cracked charge pipes, or a leaking intercooler can allow pressurized air to escape, creating a lean condition under boost.
- MAF Sensor Contamination — The mass airflow sensor measures incoming air and is sensitive to contamination from oil mist or dirt particles.
- High-Pressure Fuel Pump Weakness — The cam-driven HPFP must deliver high fuel pressure for the direct-injection system. A weak pump causes lean conditions, especially under load.
- Intake Manifold or Vacuum Leak — Cracks in the intake manifold or failing gaskets allow unmetered air into the engine.
How to Diagnose the Problem
- Read all codes and freeze frame data with an OBD-II scanner.
- Check fuel trims on Bank 2 — LTFT above +10% confirms a lean condition.
- Inspect all turbo piping, intercooler connections, and boost-related hoses for leaks.
- Pressure-test the boost system to find hidden leaks.
- Test high-pressure fuel pump output under load conditions.
- Clean the MAF sensor and compare readings to specifications.
Repair Options and Estimated Costs
- Boost leak repair: $100–$450. Replacing clamps and hoses is usually DIY-friendly.
- MAF sensor replacement: $150–$320. Easy to replace yourself.
- High-pressure fuel pump: $550–$950 with parts and labor.
- Intake manifold gasket replacement: $300–$600. Professional repair recommended on the turbo engine.
Can You Still Drive?
Avoid hard acceleration and towing. Running lean under boost creates dangerously high combustion temperatures that can damage pistons and the turbocharger. Schedule a repair within days, not weeks.
Prevention Tips
- Use premium fuel as recommended by Subaru for the 2.4L turbo.
- Follow the maintenance schedule strictly — turbo engines need more attention.
- Inspect boost tubes and intercooler piping during routine service.
- Replace the air filter on schedule to protect the MAF sensor.
- Have the fuel injectors professionally cleaned every 30,000 miles.