P0172 Code on Your 2020 Subaru Ascent: What It Means
A P0172 trouble code on your 2020 Subaru Ascent indicates that Bank 1 of the engine is running too rich. The Ascent is powered by Subaru's 2.4L FA24 turbocharged boxer four-cylinder engine, which produces 260 horsepower. When this code appears, the engine control module has determined that the air-fuel ratio on Bank 1 is consistently biased toward too much fuel, and the fuel trim corrections have exceeded normal limits.
With a family-sized SUV like the Ascent, keeping the engine running properly matters for both safety and fuel economy. Let's dig into the causes and solutions.
Symptoms You May Notice
- Check engine light on the instrument cluster
- Fuel economy dropping below the expected 21-27 MPG
- Black exhaust smoke, especially under acceleration
- Rough or unstable idle
- Turbo boost may feel inconsistent
- Sluggish acceleration or reduced power
- Strong fuel smell from the exhaust
- Possible turbo lag or surge
Common Causes of P0172 in the 2020 Ascent
The Ascent's turbocharged FA24 engine has some unique considerations compared to naturally aspirated Subarus:
- Dirty Mass Airflow Sensor (MAF) - Just like other Subaru boxer engines, the MAF sensor is the most common trigger for P0172. The turbo engine's higher crankcase pressures can push more oil vapor through the PCV system, accelerating MAF contamination. A dirty MAF underreads airflow and causes overfueling.
- Boost System Leak - The turbo system includes intercooler piping, charge pipes, and various couplers. A leak anywhere in this system allows measured air to escape after the MAF sensor, meaning less air reaches the engine than the ECM expects. The ECM then delivers too much fuel for the actual air volume.
- Faulty Upstream O2 Sensor - The Bank 1 upstream oxygen sensor controls the fuel mixture feedback loop. A biased or lazy sensor can cause the ECM to consistently add too much fuel.
- Leaking or Stuck Fuel Injectors - The FA24's direct-injection system operates at very high pressure. A leaking injector can add significant unmetered fuel to the combustion chamber, especially at idle.
How to Diagnose the Problem
Begin with an OBD-II scan to confirm P0172 and check for additional codes like P0171 (lean on Bank 2) or boost-related codes that might indicate a turbo system issue. Check fuel trims: negative STFT and LTFT values below -10% on Bank 1 confirm the rich condition. Compare Bank 1 to Bank 2 trims to see if the issue is isolated.
Inspect the MAF sensor for contamination and clean it with proper MAF cleaner. Check all turbo boost hoses, intercooler connections, and charge piping for cracks, loose clamps, or disconnected couplers. A boost leak test using a smoke machine is very effective. Monitor O2 sensor waveforms and check the purge valve operation.
Repair Options and Cost Estimates
- MAF sensor cleaning: $10-$15 for spray cleaner. Easy DIY.
- MAF sensor replacement: $140-$290 for the part, $60-$120 labor.
- Boost hose/coupler repair: $30-$150 for parts, $100-$250 labor.
- Upstream O2 sensor replacement: $100-$220 for the part, $100-$180 labor.
- Fuel injector replacement (set): $400-$800 for parts, $300-$500 labor.
- Purge valve replacement: $45-$95 for the part, $60-$130 labor.
Prevention Tips
Replace the engine air filter every 15,000 miles and more frequently if you tow or drive in dusty conditions. Use full synthetic oil as recommended for the turbo engine and change it every 6,000 miles or per the oil life monitor. Use Top Tier gasoline to keep injectors clean. Inspect turbo hoses and connections during regular service. Address any check engine lights quickly to protect the catalytic converters, which are especially expensive on the Ascent.