P0175 on the 2020 Subaru Impreza: Breaking It Down
The P0175 code on your 2020 Subaru Impreza means the engine control module has detected that bank 2 is consistently running too rich. The 2020 Impreza is powered by the 2.0-liter FB20 four-cylinder boxer engine producing 152 horsepower, paired with a Lineartronic CVT and available with Symmetrical AWD. Bank 2 on this horizontally opposed engine is the passenger side. The ECM monitors exhaust gas composition through the bank 2 upstream O2 sensor and sets this code when fuel trims show the system cannot adequately compensate for excessive fuel in the mixture.
What You Might Experience
- Check engine light on the dashboard
- Reduced fuel economy below the rated 28 city and 36 highway mpg
- Dark exhaust smoke under acceleration
- Rough or bumpy idle
- Fuel odor from the exhaust
- Possible slight hesitation during acceleration
Boxer Engine Specifics for the Impreza
The Impreza's FB20 boxer engine has the same horizontally opposed layout found across the Subaru lineup. The key difference from larger Subaru models is the smaller 2.0-liter displacement, which means the engine operates at higher relative loads during normal driving, particularly with AWD engagement. The O2 sensors on the Impreza are positioned in similar exposed locations as other Subaru boxers, making them susceptible to environmental contamination from road spray and debris.
Ranked Causes of P0175
1. Deteriorating Bank 2 O2 Sensor
At four to six years old and potentially 50,000 to 80,000 miles, the bank 2 upstream O2 sensor on the 2020 Impreza is at a prime age for degradation. The sensor tip accumulates deposits from combustion byproducts and can become sluggish in its response. When the sensor reads slowly or inaccurately, the ECM over-corrects by adding fuel, resulting in the rich condition that triggers P0175.
2. Leaking Bank 2 Fuel Injector
The Impreza's port fuel injection system uses precise injectors that must fully close between injection pulses. Carbon buildup on the injector tip or a worn internal seal can prevent complete closure, allowing fuel to drip into the intake port. This is most noticeable at idle, where the small amount of dripping fuel has a proportionally larger effect on the air-fuel ratio.
3. Fuel Pressure Regulator Malfunction
The fuel pressure regulator maintains proper rail pressure for consistent fuel delivery. If the regulator fails in a way that allows higher-than-normal pressure, every injector delivers more fuel per pulse than the ECM commands. This can cause both banks to run rich, but bank 2 may detect and report the condition first.
4. Contaminated MAF Sensor
The mass airflow sensor in the intake tract can be fouled by oil vapors from the PCV system or by dirt that bypasses the air filter. When the MAF sensor underreads actual airflow, the ECM calculates fuel delivery for less air than is present, creating a rich condition. Cleaning with dedicated MAF cleaner is the first and cheapest diagnostic step.
How to Diagnose
- Scan all codes with an OBD-II scanner and review freeze frame data
- Check bank 2 short-term and long-term fuel trims in live data mode
- Inspect air intake components, vacuum hoses, and PCV valve for leaks or damage
- Clean the MAF sensor element with MAF-specific cleaner spray
- Monitor bank 2 upstream O2 sensor voltage and switching frequency with live data
- Test fuel rail pressure with a mechanical gauge
- Perform injector flow test or leak-down test if other causes are ruled out
Cost Estimates
MAF cleaning: under $15 DIY. O2 sensor replacement: $150 to $350 at a shop. Injector cleaning service: $100 to $200. Injector replacement: $250 to $600. Fuel pressure regulator: $180 to $400. The 2020 Impreza is past the 3-year/36,000-mile basic warranty but O2 sensors may still be covered under the federal 8-year/80,000-mile emissions warranty, depending on mileage.
Impact on the Impreza's Performance
The 2.0-liter engine in the Impreza already has modest power output, so any performance loss from a rich condition is noticeable. You may feel the car is more sluggish than usual, especially merging onto highways or climbing hills. The CVT may also shift differently as it tries to compensate for the reduced engine efficiency. Getting the P0175 resolved restores the Impreza's intended responsiveness and fuel economy.