What does this code mean?
The engine needs a suitable balance of air and fuel. “Lean” means too little fuel relative to air. P0171 reports that condition on bank 1, the group containing cylinder 1. It does not simply mean the fuel tank is nearly empty.
Locate the reported problem
Bank 1 is the bank containing cylinder 1. Its physical position depends on the engine; do not assume driver side or passenger side. Confirm the engine layout before identifying hoses, sensors or cylinders.
Read it with the other codes
If P0174 is also present, both banks are involved and shared causes become important to investigate. If only P0171 is present, compare bank-specific evidence before assuming the whole engine has the same problem.
Possible causes to investigate
These are possibilities, not a diagnosis or a ranked list of failed parts.
What to do first
- Save the full code list and note whether the problem happens cold, warm, at idle or under load. Do not clear the evidence first.
- Tell the technician about recent intake work, disconnected hoses, refueling or maintenance. With the engine off and cool, only inspect safely accessible connections.
- Ask for a diagnosis using your exact year, engine and operating conditions, rather than a replacement based only on the code.
What the workshop can check
A technician can compare fuel corrections under appropriate operating conditions, check intake integrity and fuel delivery, and verify whether sensor readings are plausible. The pattern guides the next test; a single correction value is not a universal pass/fail limit.
A useful question to ask
What evidence separates an air leak from a fuel-delivery or measurement problem on bank 1?
Your vehicle changes the next step
Year, make, model, engine and the complete code list matter. Cylinder layout, test conditions and repair procedures can differ. Use service information for the exact vehicle; a generic code does not establish an exact failed part.
Can I keep driving?
This page cannot decide whether your car is safe to drive. A flashing engine light, severe shaking, overheating, strong fuel smell or major power loss calls for prompt attention. Stop safely when needed, follow your handbook and seek assistance rather than continuing a test drive.
Terms in this guide
These words appear in the explanation. Select an underlined term in the text to reach its definition.
- Lean or rich mixture
- This describes the balance of air and fuel: lean has too little fuel relative to air; rich has too much. It does not describe fuel quality.
- Bank 1 and bank 2
- A bank is a group of cylinders. Bank 1 contains cylinder 1; bank 2 is the other group, if present. Left or right cannot be assigned without identifying the engine.
- Cylinder and piston
- A cylinder is the chamber where fuel burns. A piston moves inside it and receives the force from combustion. Cylinder numbering depends on the engine.
- Control unit
- A car computer that receives measurements and controls the engine or another system. It may also be called a module, ECU, ECM or PCM.
- Oxygen or lambda sensor
- Measures oxygen in exhaust gas. Depending on its position and system, its readings help control the mixture or monitor the catalytic converter.
- Sensor and signal
- A sensor measures something such as pressure or temperature. Its signal carries that information electrically to the car computer; a wrong reading does not always mean the measured component is faulty.
- Intake
- The passages and components that bring air into the engine. The intake manifold distributes that air among the cylinders.
- PCV or crankcase ventilation
- A system routing gases from inside the engine into the intake to be burned. It is separate from the fuel-tank EVAP vent.
- Circuit, supply and ground
- A circuit includes the wiring and connections carrying electricity. Supply powers a component; ground is its electrical return. A fault in any of these can change a reading.
- Idle and rpm
- Idle is the engine running without accelerator input. Revolutions per minute, or rpm, describe how fast it turns.
- Engine load
- How much work is demanded from the engine, such as on a hill. It is not just passenger or luggage weight.
Sources and scope
Definitions and general OBD context are cross-checked against the references below. Manufacturer documents describe specific vehicles and systems; their procedures and thresholds are not instructions for every make. This is educational guidance, not a vehicle-specific repair procedure.