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P0171 – System Too Lean (Bank 1) – M271 Kompressor (Mercedes-Benz M271 1.8 Kompressor)

Fault code P0171 means "System Too Lean (Bank 1) – M271 Kompressor". It is common on M271 1.8 Kompressor (C 180 / C 200 / E 200 Kompressor, CLK 200, SLK 200), M271 CGI engines. Below are the real-world causes, step-by-step tests and OEM part numbers.

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P0171 quick summary

Fault codeP0171
MeaningSystem Too Lean (Bank 1) – M271 Kompressor
SystemPowertrain (Engine & Transmission)
VehiclesMercedes-Benz
Common onM271 1.8 Kompressor (C 180 / C 200 / E 200 Kompressor, CLK 200, SLK 200), M271 CGI
SeverityMedium
Can I drive?Yes, for a short time. A lean engine runs hotter and can misfire – have it checked soon.
Repair cost€20 – €400

Symptoms of P0171

  • Rough or hunting idle
  • Hissing noise from the engine
  • Hesitation when accelerating
  • Check Engine light
  • Sometimes P0174 or misfire codes

Most common causes of P0171 (most likely first)

Mechanic's Pro Tip

On the M271 the plastic crankcase breather hoses get brittle with age and crack, letting unmetered air in. They are cheap and the first thing to check. After that, check the boost hoses and the air mass sensor. Smoke-testing the intake finds most leaks in minutes.

  1. 1.Cracked crankcase breather hoses

    Most likely

  2. 2.Leaking intake or boost hose / intake manifold gasket

  3. 3.Dirty or faulty air mass sensor

  4. 4.Leaking supercharger bypass or vacuum line

  5. 5.Low fuel pressure

Step-by-step diagnostic guide

  1. 1

    Read all codes with XENTRY or a Mercedes-capable scanner; check long-term fuel trims.

  2. 2

    Inspect the breather hoses and boost hoses for cracks.

  3. 3

    Smoke-test the intake system.

  4. 4

    Check air mass sensor readings at idle and under load.

  5. 5

    If no leak is found, check fuel pressure.

Frequently asked questions

Why are M271 breather hoses a problem?

They are made of plastic and sit close to the hot engine. After years of heat cycles they crack, often where you cannot see it without removing parts.

How much does it cost to fix P0171 on an M271?

A set of breather hoses typically costs a few dozen euros for parts, about 100–250 € fitted. An air mass sensor costs about 80–200 €.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.