What Does P0174 Mean on Your 2020 Subaru Impreza?
If your 2020 Subaru Impreza just triggered a P0174 code, your engine's computer is telling you that Bank 2 of your 2.0L flat-four (boxer) engine is running too lean. In plain terms, there is too much air or not enough fuel reaching those cylinders. This is a fuel trim issue that needs attention before it leads to bigger problems like misfires or catalytic converter damage.
Symptoms You Might Notice
- Check engine light is on or flashing
- Rough or uneven idle, especially when cold
- Hesitation or stumbling during acceleration
- Reduced fuel economy
- Occasional stalling at stops
- A hissing sound from the engine bay (possible vacuum leak)
Common Causes of P0174 on the 2020 Impreza
The Subaru Impreza's 2.0L boxer engine uses a horizontally-opposed layout, which means vacuum leaks and intake issues can be trickier to find than on inline engines. Here are the most likely culprits:
- Vacuum Leak — Cracked or loose intake hoses, a failing PCV valve, or a leaking intake manifold gasket are the most common triggers. The boxer layout puts extra stress on intake boots.
- Faulty Mass Airflow (MAF) Sensor — A dirty or failing MAF sensor sends incorrect air volume readings to the ECU, throwing off the fuel mixture.
- Weak or Failing Fuel Pump — If fuel pressure is low, the engine cannot deliver enough fuel to match the incoming air.
- Clogged or Leaking Fuel Injectors — Injectors that are dirty or stuck can reduce fuel delivery to Bank 2 cylinders.
How to Diagnose the Problem
Start by scanning for codes with an OBD-II scanner. Check freeze frame data to see when the code set. Then follow these steps:
- Inspect all vacuum lines and intake boots for cracks, especially around the throttle body and intake manifold.
- Use a smoke machine to find hidden vacuum leaks.
- Check the MAF sensor — clean it with MAF-specific cleaner and see if readings normalize.
- Test fuel pressure at the rail. The 2020 Impreza should read around 40-58 PSI at idle.
- Check long-term fuel trim (LTFT) values. Values above +15% confirm a lean condition.
Repair Options and Estimated Costs
Repair costs vary depending on the root cause. Here is what you can expect:
- Vacuum leak repair: $100–$300 for hoses or gaskets. DIY-friendly if you can locate the leak.
- MAF sensor cleaning or replacement: $20 for cleaner, $100–$250 for a new sensor. Easy DIY job.
- Fuel pump replacement: $400–$700 including labor. Moderate difficulty for DIY.
- Fuel injector cleaning or replacement: $200–$500. Professional cleaning is recommended.
Can You Still Drive?
You can usually drive short distances with a P0174 code, but do not ignore it for long. A lean condition causes higher combustion temperatures, which can damage your catalytic converter ($1,000+ to replace) or cause engine misfires over time.
Prevention Tips
- Replace your engine air filter every 15,000–30,000 miles.
- Use quality fuel from reputable stations.
- Have your fuel system cleaned every 30,000 miles.
- Inspect vacuum hoses during every oil change for cracks or looseness.
- Do not ignore a check engine light — early diagnosis saves money.