The largest online repair guide library for every car

About us
Contact support
FAQ
Sign up
Sign in

2020 Impreza Rough Idle and Stalling at Stops - P050A, P0171

0

Mike T. · rep 0 · May 25, 2:14 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Impreza 2.0i Sport

Hey everyone, I'm having some trouble with my 2020 Subaru Impreza 2.0i Sport, which has about 45,000 miles on it. Lately, it's developed a really rough idle, especially when I'm stopped at a light or in traffic. Sometimes, it even stalls completely when I come to a stop, which is pretty scary.

I scanned the car and got two codes: P050A (Cold Idle Speed Performance) and P0171 (System Too Lean Bank 1). The car feels a bit sluggish sometimes too. I've heard about MAF sensor issues, but I'm not sure if that's what's going on here. Any advice on what I should check first?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:14 AM

0

ClearTheCode admin answer:

The combination of P050A (Cold Idle Speed Performance) and P0171 (System Too Lean Bank 1) on your 2020 Subaru Impreza points towards a significant issue with your engine's air-fuel mixture and idle control. The rough idle and stalling are direct symptoms of these underlying problems.

Possible Causes for P0171 (System Too Lean):

  1. Vacuum Leaks: This is a very common cause. A leak in any vacuum hose, intake manifold gasket, PCV valve, or brake booster can allow unmetered air into the engine, making it run lean.
  2. Mass Air Flow (MAF) Sensor: A dirty or faulty MAF sensor can send incorrect air mass readings to the engine computer, causing it to inject too little fuel.
  3. Fuel Delivery Issues: A weak fuel pump, clogged fuel filter, or dirty/failing fuel injectors can restrict fuel flow, leading to a lean condition.
  4. Exhaust Leaks: Similar to P0420, an exhaust leak before the upstream O2 sensor can cause it to read inaccurately, leading the ECU to compensate incorrectly.
  5. Faulty O2 Sensor: A failing upstream oxygen sensor could be sending incorrect data, making the engine think it's running richer than it is, thus leaning out the mixture.

Possible Causes for P050A (Cold Idle Speed Performance):

  1. Idle Air Control (IAC) Valve (if equipped): While many modern Subarus use electronic throttle bodies for idle control, if an IAC valve is present, it could be dirty or failing.
  2. Dirty Throttle Body: Carbon buildup around the throttle plate can restrict airflow at idle, leading to an unstable or low idle speed.
  3. Vacuum Leaks: As mentioned for P0171, a vacuum leak will also directly affect idle stability.
  4. Engine Management Software: Less common, but sometimes a software update can address idle performance issues.

How the Codes are Related:

A lean condition (P0171) means there's too much air or not enough fuel. This directly impacts the engine's ability to maintain a stable idle speed, especially when cold (P050A), as the engine control unit struggles to compensate for the incorrect air-fuel mixture.

When to See a Shop:

  • Immediately: Stalling at stops is a safety hazard. Driving with a lean condition can also lead to engine damage (overheating, catalytic converter damage) if not addressed promptly.

What a Shop Will Do:

  1. Diagnostic Scan and Live Data: They will analyze both codes and monitor live data from the MAF sensor, O2 sensors, fuel trims, and engine RPM to pinpoint the issue.
  2. Vacuum Leak Test: This is often the first step, using a smoke machine to visually identify leaks.
  3. MAF Sensor Cleaning/Testing: Inspect and potentially clean or test the MAF sensor.
  4. Throttle Body Cleaning: Inspect and clean the throttle body for carbon buildup.
  5. Fuel System Pressure Test: Verify proper fuel pressure and check injector performance.
  6. O2 Sensor Test: Confirm the O2 sensors are functioning correctly.
Menu

Help & account