P0175 on the 2023 Subaru Legacy: What Is Going On?
The P0175 code on your 2023 Subaru Legacy means the ECM has detected a rich air-fuel condition on bank 2 that it cannot correct through normal fuel trim adjustments. The 2023 Legacy is available with the 2.5-liter FA25 naturally aspirated boxer four-cylinder producing 182 horsepower or the 2.4-liter FA24 turbocharged boxer making 260 horsepower. Bank 2 is the passenger side of the horizontally opposed engine. The upstream O2 sensor on bank 2 provides real-time exhaust gas data that the ECM uses to maintain the optimal 14.7:1 air-fuel ratio.
Symptoms to Be Aware Of
- Check engine light illuminated
- Reduced fuel economy compared to rated figures
- Black or dark exhaust smoke
- Rough idle quality
- Fuel smell at the tailpipe
- Decreased acceleration performance
Engine Options and P0175 Differences
The diagnosis of P0175 varies slightly depending on whether your 2023 Legacy has the naturally aspirated 2.5L or the turbocharged 2.4L engine. The NA engine follows a standard diagnostic approach focusing on O2 sensors, fuel injectors, and MAF sensor. The turbo engine introduces additional variables including boost-related fuel enrichment, wastegate function, and charge air cooler integrity that can affect the air-fuel ratio on bank 2.
Most Likely Causes
1. Bank 2 O2 Sensor Issue
The upstream O2 sensor on bank 2 remains the most common cause of P0175. Even on a relatively new 2023 model, premature sensor failure from contamination, manufacturing defects, or exposure to harsh conditions can occur. The boxer engine's sensor location is lower and more exposed than on most engine configurations, contributing to potential early degradation.
2. Leaking Fuel Injectors
Fuel injectors on bank 2 can develop leaks from worn seals or carbon buildup on the pintle. The FA25 and FA24 engines use different injection systems, but both require precise fuel delivery. A dripping injector adds unmetered fuel that pushes the bank 2 mixture rich beyond the ECM's ability to compensate.
3. Fuel Pressure Running High
A stuck fuel pressure regulator allowing excessive rail pressure means every injector delivers more fuel per pulse than intended. This is a systemic issue that can affect both banks, but the bank 2 sensor configuration may detect and report it first. Connecting a fuel pressure gauge to the rail test port provides a definitive answer.
4. MAF Sensor Contamination
Oil vapor from the PCV system or fine dust bypassing the air filter can foul the MAF sensor element. The contaminated sensor underreads airflow, causing the ECM to calculate fuel delivery for less air than is actually entering the engine. Cleaning takes minutes and costs less than $15.
Step-by-Step Diagnosis
- Connect a scan tool and read all stored and pending DTCs with freeze frame data
- Monitor live data for bank 2 STFT and LTFT values at idle and 2,500 RPM
- For turbo models, check boost pressure readings and wastegate operation
- Inspect the air intake system from filter to throttle body
- Clean the MAF sensor with dedicated cleaner spray
- Evaluate bank 2 upstream O2 sensor voltage switching and response time
- Test fuel pressure at the rail against factory specifications
Warranty Coverage for the 2023 Legacy
Your 2023 Legacy is still within Subaru's 3-year/36,000-mile basic warranty and the 5-year/60,000-mile powertrain warranty. O2 sensors are also covered under the federal 8-year/80,000-mile emissions warranty. Take the vehicle to your Subaru dealer for a warranty repair rather than paying out of pocket. Attempting DIY repairs on a vehicle under warranty could complicate future claims if something goes wrong.
Cost If Paying Out of Pocket
If for any reason warranty does not apply: O2 sensor replacement runs $180 to $400, injector work costs $250 to $700, fuel pressure regulator replacement is $200 to $450, and MAF cleaning is under $15. Turbo-specific repairs can add $200 to $500 to the total depending on the issue.