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2016 Lancer Check Engine Light - Rough Idle and Bad MPG

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Mike T. · rep 0 · May 25, 4:09 AM · 1 replies · solved · Question

Vehicle: 2016 Mitsubishi Lancer ES

My 2016 Lancer ES just threw a Check Engine Light, and it's running pretty rough, especially at idle. I've also noticed my gas mileage has taken a nosedive in the last week. It feels sluggish when I accelerate too.

I scanned it with a basic OBD reader, and it gave me a P0171 code (System Too Lean Bank 1). Could this be an O2 sensor, or something more serious like a vacuum leak? Any common issues with these Lancers for this code?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:09 AM

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ClearTheCode admin answer:

A P0171 code (System Too Lean Bank 1) indicates that the engine's computer (ECU) is detecting too much air or not enough fuel in the exhaust, causing the air/fuel mixture to be leaner than optimal. This definitely aligns with your symptoms of rough idle and poor fuel economy.

Common causes for P0171 on a Mitsubishi Lancer:

  1. Vacuum Leaks: This is one of the most frequent culprits. Check all vacuum hoses, intake manifold gaskets, PCV valve, and brake booster hose for cracks, disconnections, or leaks. A simple way to check for larger leaks is to listen for a hissing sound around the engine.
  2. Mass Air Flow (MAF) Sensor: A dirty or faulty MAF sensor can send incorrect air intake readings to the ECU, leading to an improper fuel mixture. Try cleaning it with a specialized MAF sensor cleaner, but be very gentle.
  3. Oxygen Sensor (O2 Sensor): A failing upstream (pre-catalytic converter) O2 sensor can give inaccurate readings, causing the ECU to lean out the fuel mixture. While possible, it's often not the first thing to fail for a lean code.
  4. Fuel Delivery Issues:
    • Clogged Fuel Injectors: One or more injectors might not be spraying enough fuel.
    • Weak Fuel Pump: The fuel pump might not be providing adequate fuel pressure.
    • Clogged Fuel Filter: Restricts fuel flow.
  5. Exhaust Leaks: An exhaust leak before the upstream O2 sensor can allow outside air in, tricking the sensor into thinking the engine is running lean.

What to do:

  • Start by thoroughly inspecting all vacuum lines and connections. This is often the easiest and cheapest fix.
  • Consider cleaning the MAF sensor. If the problem persists, further diagnostics are needed.

When to See a Shop: If you're not comfortable with these inspections, or if cleaning the MAF sensor doesn't resolve the issue, it's best to take your Lancer to a reputable mechanic. They have specialized tools to test fuel pressure, vacuum, and O2 sensor readings accurately, which can pinpoint the exact cause of the P0171 code and get your car running smoothly again.

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