P0172 on Your 2024 Subaru Impreza: What's Going On?
Your 2024 Subaru Impreza's check engine light is on with a P0172 code, indicating that the fuel system on Bank 1 is running too rich. The 2024 Impreza features the updated 2.5L FB25 boxer four-cylinder engine, and its ECM has detected that the air-fuel ratio is skewed toward too much fuel. The computer has been trying to reduce fuel delivery to compensate, but corrections have exceeded the normal operating range.
Even though this is a newer vehicle, a P0172 code can still appear. Let's look at why this happens and what you can do about it.
Symptoms You May Notice
- Check engine light on the dashboard
- Reduced fuel economy below the expected 28-36 MPG
- Dark or black exhaust smoke
- Rough or unsteady idle
- Sluggish throttle response
- Fuel smell from the exhaust area
- Potential misfires at idle or low RPM
Common Causes of P0172 in the 2024 Impreza
Even newer vehicles like the 2024 Impreza can develop rich-running issues. Here are the most common causes in order of likelihood:
- Contaminated Mass Airflow Sensor (MAF) - Even on a new vehicle, the MAF sensor can become contaminated by oil vapor from the crankcase ventilation system or from an over-oiled aftermarket air filter. When contaminated, it underreads airflow and the ECM adds too much fuel.
- Software or Calibration Issue - Being a newer model year, the 2024 Impreza may have ECM calibration updates available from Subaru that address fuel trim mapping issues. Check with your dealer for any applicable TSBs or software updates.
- Faulty Upstream O2 Sensor - While uncommon on a vehicle this new, a defective O2 sensor out of the box or one damaged by contamination can send incorrect readings to the ECM, causing rich fueling.
- EVAP Purge Valve Issue - A purge valve that fails to close properly sends unmetered fuel vapors into the intake manifold, enriching the mixture beyond what the ECM expects.
How to Diagnose the Problem
Since this is a 2024 model, your first step should be checking with your Subaru dealer. The vehicle is likely still under the 3-year/36,000-mile bumper-to-bumper warranty or the 5-year/60,000-mile powertrain warranty. Any emissions-related repairs are covered under the 8-year/80,000-mile federal emissions warranty.
If you want to investigate yourself, use an OBD-II scanner to check fuel trim values. Negative STFT and LTFT values below -10% on Bank 1 confirm the rich condition. Check for any TSBs related to P0172 for the 2024 Impreza. Inspect the MAF sensor and air filter for contamination. On a car this new, avoid extensive DIY repairs that might affect warranty coverage.
Repair Options and Cost Estimates
- Warranty repair at dealership: $0 if covered under warranty (most likely for a 2024 model).
- MAF sensor cleaning: $10-$15 for spray cleaner. Quick DIY job, will not void warranty.
- MAF sensor replacement: $120-$250 for the part, $50-$100 labor.
- ECM software update: $0 under warranty, $100-$200 at dealer if out of warranty.
- Upstream O2 sensor replacement: $80-$180 for the part, $80-$150 labor.
- Purge valve replacement: $45-$90 for the part, $60-$110 labor.
Prevention Tips
Use the factory air filter or a high-quality OEM replacement. If using a reusable oiled air filter, apply oil sparingly to avoid MAF contamination. Always use Top Tier gasoline. Keep up with all scheduled maintenance at your Subaru dealer, especially during the warranty period. Don't ignore software update notifications from Subaru, as these can address driveability and emissions issues.