P0171

What it means

P0171 means the engine control unit has found that Bank 1 is running too lean. The fuel trim corrections needed to hold the target air-fuel ratio have gone beyond their allowed limit, which tells the ECU there is too much air or too little fuel reaching the cylinders. This is the generic SAE J2012 definition, and the underlying cause varies with engine and model.

Symptoms

• Malfunction indicator lamp on
• Rough or unstable idle
• Hesitation or lack of power under acceleration
• Occasional misfire or stumbling at light load
• Poor fuel economy
• Hard starting or occasional stalling

Possible causes

• Unmetered air entering after the air flow sensor, such as a split intake hose, loose clip or leaking intake manifold gasketlikely
• Leaking or disconnected vacuum hose, brake servo hose or crankcase ventilation hoselikely
• Contaminated or faulty mass air flow sensor reading lower than actual airflowlikely
• Low fuel pressure from a weak pump, blocked filter (where fitted as a service item) or faulty pressure regulatorpossible
• Blocked or restricted fuel injector on Bank 1possible
• Exhaust leak upstream of the front oxygen sensor letting in air and giving a false lean readingpossible
• Faulty upstream oxygen or air-fuel ratio sensor, or a fault in its wiringrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data (engine load, temperature, fuel trims, speed) to see when the fault occurs.
2. Check live short-term and long-term fuel trims at idle and at about 2500 rpm. A lean condition that improves at higher revs points towards a vacuum or air leak, while one that worsens points towards fuel delivery or airflow measurement.
3. Visually inspect the intake pipework, air filter housing, vacuum hoses, crankcase ventilation hoses and electrical connectors for splits, loose fittings and damage.
4. Check for air leaks with a smoke test of the intake system, and listen for hissing or check for a change in idle when suspect areas are disturbed.
5. Compare the mass air flow sensor reading with the expected value at idle and check the sensor element for contamination, following the manufacturer's procedure.
6. Measure fuel pressure and check its stability against the specification for the model and engine.
7. Inspect the exhaust upstream of the front oxygen sensor for leaks, and check the sensor's live response and wiring.
8. Check injector operation and spray or balance on the affected bank, using the procedure appropriate to the engine.

How to fix

• Replace or refit any split, loose or disconnected intake hoses, vacuum hoses or crankcase ventilation hoses.
• Replace leaking intake manifold or throttle body gaskets and seals.
• Clean or replace the mass air flow sensor if it is contaminated or out of range.
• Repair the fuel supply fault found, such as a weak pump, blocked filter or faulty regulator, to restore correct pressure.
• Clean or replace faulty injectors on Bank 1.
• Repair exhaust leaks ahead of the oxygen sensor, or replace a faulty sensor or damaged wiring, then confirm fuel trims return to normal.

Can I keep driving?

This code for other makes

• P0171AudiControl module software needs to be updated, Vacuum leaks (intake manifold gaskets, vacuum hoses, PCV hoses, etc.), Mass air flow sensor, Plugged fuel filter or weak fuel pump, Plugged or dirty fuel injectors
• P0171BuickSystem Lean Bank 1

Nearby codes

• P0170GenericFuel Trim Bank 1
• P0172GenericSystem Too Rich Bank 1