P0175 on the 2020 Subaru Ascent: What Is Happening?
The P0175 code on your 2020 Subaru Ascent means the ECM has detected a sustained rich air-fuel mixture on bank 2 that exceeds its correction capability. The Ascent is powered exclusively by the 2.4-liter FA24 turbocharged boxer four-cylinder producing 260 horsepower, the same engine used in the Legacy and Outback turbo trims. Bank 2 is the passenger side of the horizontally opposed engine. As Subaru's largest three-row SUV, the Ascent works its turbo engine harder than smaller Subaru models, especially when loaded with passengers and cargo.
Common Symptoms
- Check engine light on
- Fuel economy below the rated 21 city and 27 highway mpg
- Black or dark exhaust smoke, especially under boost
- Rough or unsteady idle
- Fuel smell from the exhaust
- Reduced power or turbo lag
- Possible boost-related hesitation
Turbo Boxer Engine Specifics
The Ascent's 2.4-liter turbo boxer engine presents unique P0175 considerations compared to naturally aspirated Subaru models. During boost, the ECM intentionally enriches the fuel mixture to prevent detonation and protect the engine. If the turbo system has an issue, such as a wastegate that does not regulate boost properly or a leaking charge air cooler, the ECM may add excessive fuel enrichment that pushes bank 2 over the threshold. Additionally, the Ascent's heavier curb weight of approximately 4,500 pounds means the turbo is called upon more frequently, increasing the demands on the fuel system.
Primary Causes
1. Degraded Bank 2 Upstream O2 Sensor
At four to six years old, the bank 2 upstream O2 sensor on the Ascent can deteriorate from exhaust heat exposure and combustion byproducts. The turbo engine produces hotter exhaust gases than a naturally aspirated engine, which accelerates sensor wear. A sluggish or biased sensor causes the ECM to over-fuel bank 2.
2. Fuel Injector Leak or Malfunction
The Ascent's direct injection system operates at very high pressures to handle the turbo engine's fuel demands. An injector on bank 2 that leaks or does not close properly adds unmetered fuel to the combustion chamber. Under boost, this extra fuel is even more significant because the engine is already receiving enrichment from the turbo calibration.
3. Boost System Issue Causing Over-Enrichment
Problems with the wastegate actuator, boost control solenoid, or charge air cooler can cause the turbo to produce more boost than expected. The ECM responds by adding more fuel for detonation protection, which can push bank 2 into a sustained rich condition. A boost pressure test and wastegate inspection are important diagnostic steps on the turbo Ascent.
4. Contaminated MAF Sensor
The MAF sensor on the Ascent sits upstream of the turbocharger in the intake tract. If contaminated by oil mist or debris, it underreads airflow and causes the ECM to inject less fuel than the engine actually needs at off-boost conditions. But during boost, the turbo adds additional air that the dirty MAF does not account for, creating inconsistent fuel trims that can trigger P0175.
Diagnostic Steps
- Scan for all codes and review freeze frame data, noting boost pressure and engine load conditions
- Monitor bank 2 STFT and LTFT at idle, light cruise, and under boost
- Check boost pressure readings against factory specifications
- Inspect the charge air cooler piping and turbo inlet for leaks
- Clean the MAF sensor with dedicated MAF cleaner spray
- Monitor bank 2 upstream O2 sensor waveform for proper switching
- Test fuel pressure at the rail under various conditions
- Inspect the wastegate actuator for proper operation
Repair Costs
O2 sensor replacement: $200 to $450. Fuel injector work: $300 to $800. Boost system repairs: $200 to $600. MAF cleaning: under $15. The 2020 Ascent is past the basic warranty period but the federal emissions warranty covers O2 sensors for 8 years or 80,000 miles. Check your mileage to see if you qualify.
AWD and Towing Considerations
The Ascent's Symmetrical AWD and 5,000-pound towing capacity mean the engine works hard under real-world conditions. Towing with a rich condition is not recommended as it compounds the catalytic converter risk and further reduces the already-lower towing fuel economy. Fix the P0175 before your next towing trip.