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P0171

P0171 on a 2016 Mercedes Benz C Class Phev

P0171 means the engine computer is adding more fuel than normal on bank 1 because it sees too much air or not enough fuel.

Not your vehicle? See general P0171 info.

Most common causes on the 2016 Mercedes Benz C Class Phev

Ranked from 317 NHTSA owner complaints filed for this vehicle.

Sort by:
  1. 1

    Fuel supply line/hose leak upstream of the injectors (recall-related)

    High severity

    Dozens of owners describe a strong gas smell and visible fuel leaking from a cracked low-pressure fuel hose or loose compression clamp (subject of an extended-warranty campaign for fuel line leaks); a leak here drops rail pressure and can pull the mixture lean.

    Reported in 11 NHTSA complaints
    Parts:fuel linefuel line clamp
  2. 2

    PCV/crankcase vent valve stuck or leaking (P052E)

    Medium severity

    A stuck-open PCV/crankcase regulator valve is a textbook cause of a lean fuel-trim code, and this exact valve is the single most-repeated complaint in this population.

    Reported in 9 NHTSA complaints
    Parts:PCV valve
  3. 3

    Vacuum or intake air leak

    Low severity

    A cracked hose or loose intake boot admits unmetered air, pulling bank 1 lean, especially noticeable at idle.

    Common cause
    Parts:intake hosevacuum hose
  4. 4

    Dirty or failing MAF sensor

    Low severity

    A contaminated or drifting mass airflow sensor under-reports incoming air, causing the ECU to under-fuel.

    Common cause
    Parts:MAF sensor
  5. 5

    Exhaust leak upstream of the front O2 sensor

    Medium severity

    Outside air drawn in through a manifold or flange leak skews the upstream O2 reading lean.

    Common cause
    Parts:exhaust gasket
  6. 6

    Intake manifold gasket failure

    Medium severity

    A failed manifold gasket is a common unmetered-air path on aging V6/I4 engines and reads as a lean condition.

    Common cause
    Parts:intake manifold gasket

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0171 on your car still needs shop confirmation before any repair.

Common symptoms

  • Check engine light
  • Rough idle
  • Hesitation
  • Surging idle
  • Poor fuel economy

What to check before replacing parts

1

Review short- and long-term fuel trims on bank 1

Look for: High positive trims (commonly worse at idle, improving with RPM) mean the computer is adding fuel to compensate for excess air or insufficient fuel — that pattern is this code's fingerprint.

2

Smoke-test the intake

Look for: Smoke escaping (or a hiss you can hear) at intake boots, gaskets, or vacuum hose connections confirms unmetered air is entering after the MAF sensor, which is the most common cause of a lean code.

3

Inspect the PCV hoses and valve

Look for: A cracked, split, or disconnected PCV hose, or a PCV valve stuck open, draws in extra unmetered air the same way a vacuum leak does.

4

Clean and test the MAF sensor

Look for: Visible dirt or oil film on the sensor element, or a grams/second reading that doesn't track expected airflow for RPM, means the MAF is under-reporting air and causing the computer to under-fuel.

5

Verify fuel pressure

Look for: Pressure below spec (weak pump, clogged filter, failing regulator) starves the engine of fuel and produces the same lean trim pattern as a vacuum leak — rule it out before chasing leaks.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Car Runs Rough