What Does P0174 Mean on Your 2024 Subaru WRX?
When your 2024 Subaru WRX displays a P0174 code, it means the engine control module has detected that Bank 2 of your FA24 2.4L turbocharged boxer engine is running too lean. The ECM has added as much extra fuel as it can, and the mixture is still too far on the air-heavy side.
Symptoms You Might Notice
- Check engine light illuminated or flashing
- Rough or hunting idle
- Reduced turbo boost and sluggish acceleration
- Worse fuel economy
- Engine pinging or knocking under acceleration
- Whistling or hissing sounds from the engine bay
Common Causes of P0174 on the 2024 WRX
The 2024 WRX continues with the FA24 2.4L turbo engine making 271 hp. Here are the most likely causes of a lean condition:
- Boost Leak — The turbo inlet, charge pipe, intercooler piping, or bypass valve can develop leaks. This is the most common cause on turbocharged WRX models.
- MAF Sensor Problem — A contaminated or malfunctioning mass airflow sensor sends incorrect readings, causing the ECM to under-fuel Bank 2.
- High-Pressure Fuel Pump Issue — The direct-injection HPFP must maintain high pressures under boost. A weak pump cannot keep up, resulting in lean conditions.
- Faulty Bank 2 O2 Sensor — A slow or biased oxygen sensor on Bank 2 can falsely report lean conditions, prompting the ECM to set P0174.
How to Diagnose the Problem
- Scan all codes and review freeze frame data for engine load and RPM at the time of the code.
- Monitor Bank 2 fuel trims at idle and under boost. LTFT above +10% confirms the lean issue.
- Inspect all boost connections, intercooler piping, and the bypass valve.
- Pressure-test the boost system using a leak-down tester.
- Test HPFP fuel pressure under load with an advanced scan tool.
- Clean or replace the MAF sensor as needed.
Repair Options and Estimated Costs
- Boost leak repair: $80–$400. Replacing clamps and couplers is straightforward.
- MAF sensor replacement: $150–$300. Simple bolt-on replacement.
- High-pressure fuel pump: $550–$950 including labor.
- O2 sensor replacement: $180–$400. Moderate difficulty.
Can You Still Drive?
Drive gently and stay out of boost. A lean condition under high boost on the WRX turbo engine risks detonation, which can crack pistons or destroy the turbo. Get this diagnosed and repaired within days.
Prevention Tips
- Always use premium 93 octane fuel as required by the WRX.
- If you have aftermarket modifications, ensure you have a proper professional tune.
- Inspect turbo piping and intercooler connections at every service.
- Keep the air filter clean and avoid over-oiling reusable filters.
- Monitor fuel trims with a scan tool or Cobb AccessPort regularly.