What Does P0172 Mean on a 2020 Subaru Legacy?
If your 2020 Subaru Legacy has triggered a P0172 code, the engine is running rich on Bank 1. The 2020 Legacy was the first year of the current generation and is available with either a 2.5L FB25 naturally aspirated boxer four-cylinder or the more powerful 2.4L FA24 turbocharged boxer engine. Regardless of which engine your Legacy has, a P0172 code means the ECM has detected that the fuel mixture is consistently too rich and its fuel trim corrections have exceeded the normal operating range.
Addressing this issue promptly helps prevent catalytic converter damage and keeps your Legacy running efficiently.
Symptoms You May Notice
- Check engine light illuminated
- Decreased fuel economy (normally 27-32 MPG for the 2.5L, 24-32 for the 2.4T)
- Black smoke from the exhaust
- Rough or unstable idle
- Engine hesitation or lack of power
- Noticeable fuel smell at the exhaust
- Possible misfires or rough running
Common Causes of P0172 in the 2020 Legacy
Both the 2.5L and 2.4L turbo boxer engines share similar potential causes for a rich-running condition:
- Dirty or Failing Mass Airflow Sensor (MAF) - The MAF sensor is the most common trigger for P0172 in Subaru boxer engines. Contamination from oil vapor, dust, or oiled aftermarket air filters causes it to underreport air volume, leading to excess fuel delivery.
- Faulty Upstream O2 Sensor (Bank 1) - A degraded or biased oxygen sensor can provide incorrect readings to the ECM, causing the fuel system to run rich. On the Legacy's boxer layout, Bank 1 sensors are on the passenger side.
- Leaking Fuel Injectors - Both the FB25 and FA24 engines use direct injection. Worn injector tips or stuck-open injectors allow excess fuel into the cylinders. The turbo FA24 runs at even higher fuel pressures, making injector issues more impactful.
- Boost Leak or Wastegate Issue (2.4L Turbo only) - On the turbocharged model, a boost leak or stuck wastegate can cause the ECM to miscalculate airflow and over-fuel the engine. Check all charge piping connections and intercooler hoses.
How to Diagnose the Problem
Start with an OBD-II scan to confirm P0172 and check for companion codes. Check fuel trim data, looking for negative STFT and LTFT values below -10% on Bank 1. For the turbocharged model, also monitor boost pressure readings. Inspect the MAF sensor for contamination and try cleaning it with MAF-specific spray.
On the turbo model, inspect all boost hoses, the intercooler connections, and the bypass valve for leaks. You can use a smoke machine to find boost leaks in the charge piping. For both engines, check the PCV system, inspect the purge valve, and monitor O2 sensor waveforms with live data.
Repair Options and Cost Estimates
- MAF sensor cleaning: $10-$15 for spray cleaner. Quick DIY job.
- MAF sensor replacement: $130-$270 for the part, $60-$120 labor.
- Upstream O2 sensor replacement: $90-$200 for the part, $90-$160 labor.
- Fuel injector replacement: $300-$600 for parts (set), $200-$400 labor. Higher end for the turbo model.
- Boost leak repair (turbo): $20-$150 for hoses/clamps, $100-$200 labor.
- Purge valve replacement: $40-$85 for the part, $60-$120 labor.
Prevention Tips
Replace the engine air filter on schedule, every 15,000-20,000 miles. Avoid over-oiling reusable air filters. Use Top Tier gasoline to minimize injector deposits. On the turbo model, use the recommended synthetic oil and follow the more frequent oil change interval to keep the turbo and PCV system clean. Address any check engine lights promptly to avoid catalytic converter damage.