What Does a P0172 Code Mean on Your 2023 Subaru Ascent?
If your 2023 Subaru Ascent has set a P0172 code, the engine control module is reporting that Bank 1 of the 2.4L FA24 turbocharged boxer engine is running too rich. This means the air-fuel ratio is skewed toward excess fuel. The ECM has attempted to correct this by pulling back on fuel delivery, but the corrections have exceeded the acceptable threshold, triggering the check engine light.
The good news is that your 2023 Ascent should still be under warranty. Let's look at the likely causes and your options for getting it resolved.
Symptoms You May Notice
- Check engine light illuminated
- Fuel economy dropping below the typical 21-27 MPG range
- Black smoke visible from the exhaust
- Rough idle or engine shaking at stoplights
- Reduced power or turbo boost inconsistency
- Fuel odor from the tailpipe area
- Engine running rougher than normal, especially during warmup
Common Causes of P0172 in the 2023 Ascent
The 2023 Ascent's turbocharged FA24 engine shares its architecture with other Subaru turbo models. Here are the most likely causes:
- Contaminated Mass Airflow Sensor (MAF) - The MAF sensor is the leading cause of P0172 across Subaru's boxer engine lineup. The turbo engine's positive crankcase ventilation system can deposit oil film on the MAF sensing element, causing it to underread airflow and trigger overfueling.
- ECM Software or Calibration Update Needed - As a newer model, the 2023 Ascent may benefit from ECM recalibration. Subaru periodically releases software updates that refine fuel trim strategies. Your dealer can check for applicable TSBs.
- Turbo Charge Piping Leak - Any leak in the intercooler piping, charge hoses, or boost plumbing allows metered air to escape. The ECM doesn't know air has been lost, so it delivers fuel based on the original air measurement, creating a rich condition.
- Faulty Upstream Oxygen Sensor - Although uncommon on a vehicle this new, a defective O2 sensor can send biased voltage readings to the ECM, causing it to add unnecessary fuel.
How to Diagnose the Problem
Since your 2023 Ascent is almost certainly still under warranty coverage, the best course of action is to bring it to your Subaru dealer. The 3-year/36,000-mile bumper-to-bumper warranty and the 8-year/80,000-mile federal emissions warranty both apply to this issue.
If you want to do some preliminary checks, scan for codes and review fuel trim data. Negative Bank 1 STFT and LTFT values below -10% confirm the rich condition. You can safely clean the MAF sensor with proper MAF spray cleaner without affecting warranty coverage. Check for any obvious boost leaks by looking at charge pipe connections and listening for hissing sounds under boost.
Repair Options and Cost Estimates
- Warranty repair at dealer: $0 under bumper-to-bumper or emissions warranty.
- MAF sensor cleaning: $10-$15 for spray. DIY-friendly, doesn't void warranty.
- MAF sensor replacement: $150-$300 for the part, $60-$120 labor.
- ECM software update: $0 under warranty, $100-$200 if paying out of pocket.
- Boost piping repair: $30-$180 for parts, $100-$250 labor.
- O2 sensor replacement: $100-$220 for the part, $100-$180 labor.
Prevention Tips
Use a quality OEM or equivalent air filter and replace it every 15,000 miles. The turbo engine benefits from more frequent oil changes with full synthetic oil to minimize PCV contamination. Use Top Tier gasoline. Don't neglect scheduled maintenance, and inspect turbo hoses and connections during service visits. Address check engine lights promptly to prevent catalytic converter damage, which is especially costly on the Ascent.