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2018 C300 Check Engine Light - P0171/P0174 Lean Codes

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Chris P. · rep 0 · May 22, 4:13 AM · 1 replies · solved · Question

Vehicle: 2018 Mercedes-Benz C300 Sedan

Hey everyone,

My 2018 C300 (W205) with the M274 engine just threw a check engine light. I scanned it and got P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2). The car feels a bit rough at idle, and I've noticed a slight dip in fuel economy lately. It's got about 65,000 miles on it.

Has anyone experienced this with their C300? What was the culprit? I'm hoping it's something relatively simple and not a major engine issue. Any advice on where to start troubleshooting would be greatly appreciated before I take it to the dealer.

Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:13 AM

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ClearTheCode admin answer:

Lean condition codes (P0171/P0174) on your 2018 Mercedes-Benz C300 are common and typically point to unmetered air entering the engine or insufficient fuel delivery. Here's a breakdown of common causes and steps:

  1. Inspect for Vacuum Leaks:

    • Causes: Cracked vacuum lines, intake manifold gaskets, PCV (Positive Crankcase Ventilation) valve or its hoses, or a loose air intake boot after the MAF sensor. A smoke test is often the most effective way for a shop to find these.
    • Diagnostics: Listen for hissing sounds around the engine bay. Visually inspect all rubber and plastic hoses connected to the intake manifold.

  2. Check MAF (Mass Air Flow) Sensor:

    • Causes: A dirty or failing MAF sensor can send incorrect air readings to the ECU, leading to an incorrect fuel mixture.
    • Diagnostics: Try cleaning the MAF sensor with a specialized MAF cleaner. If the problem persists, the sensor may need replacement.

  3. Fuel System Issues:

    • Causes: Clogged fuel filter, weak fuel pump, or dirty fuel injectors can restrict fuel flow.
    • Diagnostics: This usually requires pressure testing the fuel system and checking injector spray patterns, which is best done by a professional.

  4. Oxygen Sensors:

    • Causes: Faulty upstream oxygen sensors can provide incorrect readings, causing the ECU to lean out the mixture.
    • Diagnostics: Live data from a diagnostic scanner can show O2 sensor voltage readings. If they are stuck low, it could indicate a problem.

When to See a Shop: If you've checked the basic items like vacuum lines and MAF sensor cleaning without success, or if you're uncomfortable performing these checks, it's highly recommended to take your C300 to a qualified Mercedes-Benz specialist. They have the advanced diagnostic tools (like XENTRY/DAS) to pinpoint the exact cause, perform smoke tests, and accurately diagnose fuel system or sensor failures. Driving with a persistent lean condition can lead to long-term engine damage, including catalytic converter failure.

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