Credov

P0171 fault code: what it means

P0171: Bank 1 is running lean (too much air or not enough fuel) and the engine computer has run out of correction.

Engine and gearbox (powertrain) · Costly if missed

Usual causes, most likely first

  1. vacuum or intake air leak (hose, gasket, PCV)
  2. dirty or faulty MAF sensor
  3. weak fuel pump or blocked fuel filter
  4. exhaust leak before the front O2 sensor

P0171 by make

Audi

Engine running lean on bank 1 (unmeasured air getting in). On TSI/TFSI the torn PCV/crankcase breather diaphragm is the number one cause.

Usual causes: torn PCV / crankcase breather valve diaphragm; vacuum or intake leak (hoses, manifold seals, brake booster line); boost pipe leak (turbo engines); MAF sensor under-reading

BMW

Engine running lean, bank 1 (cylinders 1-3 on a straight six). On BMW the crankcase vent valve membrane in the rocker cover is a classic cause, worse at cold idle.

Usual causes: split crankcase ventilation (PCV) valve membrane; intake or vacuum leak (boots, oil filler cap); low fuel pressure (older N52 / N46); air mass meter fault

Ford

Engine running lean (bank 1): too much air or not enough fuel. Common causes are unmetered air leaks.

Usual causes: vacuum or PCV hose leak; split intake duct; dirty or faulty MAF sensor; low fuel pressure

Haval

System too lean.

Usual causes: vacuum or PCV leak; boost leak; weak fuel supply; dirty air flow reading

Holden

Engine running lean, bank 1: too much air or not enough fuel. On Commodore V6 usually an air leak after the airflow meter.

Usual causes: vacuum, PCV or intake duct leak; leaking intake manifold gasket; dirty or faulty MAF sensor; low fuel pressure

Honda

Engine running lean (bank 1): too much air or too little fuel. On Honda K-series and 1.5 turbo, air leaks at the intake manifold and PCV are the usual cause.

Usual causes: intake manifold gasket or cracked manifold leak; split PCV or vacuum hose; dirty MAF sensor; weak fuel delivery

Hyundai

Engine running lean (too much air, not enough fuel) on bank 1.

Usual causes: vacuum leak (cracked hose, PCV hose or valve); dirty or faulty MAF sensor; intake manifold/plenum gasket leak (V6); low fuel pressure

Jeep

Fuel system too lean, bank 1.

Usual causes: vacuum leak (intake gasket, PCV hose); dirty MAF/air flow reading; weak fuel pump or blocked filter; exhaust leak before the O2 sensor

Kia

Engine running lean (bank 1).

Usual causes: vacuum leak; faulty oxygen sensor; dirty injectors or low fuel pressure; oil burning (Theta II / Nu) skewing fuel trims

Lexus

Fuel mixture too lean (bank 1): the ECM is adding lots of fuel to correct for extra air.

Usual causes: vacuum or intake leak after the air flow meter; dirty air flow meter; weak fuel pump or blocked injectors

Mazda

Fuel trim too lean. On SkyActiv-G direct-injection engines check for air leaks first, then MAF sensor and the high-pressure fuel pump.

Usual causes: intake air leak or split vacuum/PCV hose; dirty or faulty MAF sensor; low fuel pressure (high-pressure pump or fuel pressure sensor); fuel leak in fuel line

Mercedes-Benz

Engine running lean, bank 1. On Mercedes petrols usually an air-metering or vacuum leak problem.

Usual causes: dirty or faulty MAF sensor; vacuum or PCV leak; clogged fuel injectors

MG

System too lean.

Usual causes: vacuum or PCV leak; dirty air flow reading; weak fuel supply

Mini

Engine running lean. On Mini (BMW-designed engines) a split crankcase vent (PCV) valve or rocker cover leak is a usual cause, worst at cold idle.

Usual causes: crankcase vent (PCV) valve or rocker cover leak; intake or vacuum hose leak; low fuel pressure; injector fault

Mitsubishi

Engine running lean (too much air).

Usual causes: vacuum leak (hoses, intake gasket, PCV); dirty MAF sensor; weak fuel pump or blocked filter; faulty front O2 sensor

Nissan

Engine running lean on bank 1. On VQ V6s with P0174 it points to something affecting the whole engine.

Usual causes: vacuum leak (cracked PCV/breather hose or intake boot); dirty MAF sensor; intake manifold or plenum gasket leak

Skoda

Engine running lean on bank 1 (unmeasured air getting in). On TSI/TFSI the torn PCV/crankcase breather diaphragm is the number one cause.

Usual causes: torn PCV / crankcase breather valve diaphragm; vacuum or intake leak (hoses, manifold seals, brake booster line); boost pipe leak (turbo engines); MAF sensor under-reading

Subaru

Engine is running lean (too much air). On FB25 Foresters usually the front A/F sensor or a cracked PCV/vacuum hose.

Usual causes: failing front air-fuel ratio sensor; cracked PCV hose or other vacuum leak; dirty MAF sensor; weak fuel pump or blocked fuel filter

Suzuki

System too lean.

Usual causes: vacuum leak; dirty MAF/MAP reading; weak fuel pump or blocked filter; exhaust leak before O2 sensor

Toyota

Fuel mixture too lean (bank 1): the ECM is adding lots of fuel to correct for too much air.

Usual causes: vacuum or intake leak after the air flow meter (intake manifold gasket on 2AZ Camry); dirty air flow meter; weak fuel pump or blocked injectors

Volkswagen

Engine running lean on bank 1 (unmeasured air getting in). On TSI/TFSI the torn PCV/crankcase breather diaphragm is the number one cause.

Usual causes: torn PCV / crankcase breather valve diaphragm; vacuum or intake leak (hoses, manifold seals, brake booster line); boost pipe leak (turbo engines); MAF sensor under-reading

Volvo

System too lean, bank 1. On Volvo 5-cylinder petrols a blocked PCV oil trap is a classic cause.

Usual causes: blocked PCV oil trap / split PCV hose; vacuum leak at intake; dirty MAF; weak fuel pump

What the repair usually costs

Not sure what P0171 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.