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CEL P0171 on 2017 Impreza - System Too Lean

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Chris P. · rep 0 · May 22, 3:18 AM · 1 replies · solved · Question

Vehicle: 2017 Subaru Impreza 2.0i Premium

Hey everyone,

My 2017 Subaru Impreza 2.0i Premium just threw a Check Engine Light (CEL) and when I scanned it, the code came up as P0171: System Too Lean (Bank 1). The car seems to be running okay, maybe a slight hesitation sometimes, but nothing major. Fuel economy might be a little worse than usual.

I'm not sure where to start looking. Could it be an oxygen sensor, or something else? I've heard this code can be tricky to diagnose. Any common culprits for this on a 2017 Impreza?

Any advice on what I should check first before taking it to a mechanic would be great. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:18 AM

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ClearTheCode admin answer:

The P0171 diagnostic trouble code (DTC) indicates that the engine's computer (ECU) has detected a lean condition in Bank 1, meaning there's too much air or not enough fuel in the air/fuel mixture. While your 2017 Subaru Impreza might seem to run okay, a persistent lean condition can lead to long-term engine damage and reduced fuel efficiency.

Common Causes for P0171:

  1. Vacuum Leaks: This is one of the most common causes. Leaks in vacuum lines, intake manifold gaskets, PCV valve, or brake booster can introduce unmetered air into the engine.
  2. Mass Air Flow (MAF) Sensor: A dirty or faulty MAF sensor can send incorrect air mass readings to the ECU, leading it to inject less fuel than needed.
  3. Oxygen (O2) Sensor: A failing upstream (pre-catalytic converter) O2 sensor can provide inaccurate readings, causing the ECU to incorrectly adjust fuel trim.
  4. Fuel Delivery Issues: This includes a weak fuel pump, clogged fuel filter, or dirty/failing fuel injectors that aren't delivering enough fuel.
  5. Exhaust Leaks: Leaks before the upstream O2 sensor can allow outside air in, tricking the sensor into reading a lean condition.

Troubleshooting Steps:

  1. Inspect for Vacuum Leaks: Visually check all vacuum hoses, the intake manifold, and PCV valve for cracks, disconnections, or signs of wear. You can also listen for a hissing sound around the engine.
  2. Clean the MAF Sensor: Carefully remove the MAF sensor (located in the air intake tube after the air filter) and clean it with a dedicated MAF sensor cleaner. Do NOT use other solvents.
  3. Check Air Filter: Ensure your air filter is clean and not clogged, which can affect airflow.
  4. Inspect O2 Sensor Wiring: Look for any damaged or corroded wiring leading to the upstream O2 sensor.

When to See a Shop:

If after checking for vacuum leaks and cleaning the MAF sensor the P0171 code returns, it's highly recommended to take your Impreza to a professional mechanic or Subaru dealership. They have specialized diagnostic equipment to perform smoke tests for vacuum leaks, monitor live data from the O2 and MAF sensors, and test fuel pressure and injector performance. This will ensure an accurate diagnosis and prevent potential damage from prolonged lean running conditions.

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