P0171: System Too Lean (Bank 1)

What P0171 means

P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 1, places it in the fuel and air metering subsystem; the last two digits identify the specific fault.

Fuel trim is how much the engine computer is correcting its base fuel calculation, using the upstream O2 sensors as feedback. Positive trim means it is adding fuel to correct a lean mixture; negative means it is removing fuel to correct a rich one. These codes set when the correction reaches its limit.

Bank 1 is the side of the engine that contains cylinder 1. Inline engines have only bank 1, so a bank 2 code on an inline engine usually indicates a manufacturer-specific use.

What “System Too Lean” means here

The module has added as much fuel as it is allowed and the upstream O2 sensor still reads lean. Something is letting in air the engine is not measuring (vacuum leak, intake leak after the MAF, exhaust leak ahead of the sensor) or not delivering enough fuel (weak pump, clogged filter, restricted injectors), or the MAF is under-reporting.

What you may notice

Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0171:

• Hesitation or rough idle
• Higher fuel use
• Misfire codes on some engines
• Fuel smell or black smoke when rich

Causes to consider

• Intake vacuum leak
• Dirty MAF sensor
• Weak fuel pump
• Clogged fuel injectors

Where to start diagnosing P0171

1. Read short and long term fuel trims at idle and at 2,500 rpm. Trims that are high at idle but improve with rpm point to a vacuum leak; trims that are high everywhere point to fuel delivery or air metering.
2. Check for vacuum and intake leaks, including PCV hoses.
3. Check MAF readings and fuel pressure.
4. If both banks are affected, look for a shared cause; if only one, look at that bank.

Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.

Live data to watch

These standard OBD2 values relate to P0171. Support varies by vehicle; a value your car does not report will simply be missing.

Related codes

Codes from the same fuel trim system family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.

Further reading

• Your fuel economy fell off a cliff
• Fuel trim mastery: reading STFT and LTFT
• O2 sensor switching and waveform analysis

Symptoms that lead to P0171

If you have not scanned the car yet, or P0171 is one of several codes stored, these guides start from what the vehicle is actually doing.

• Why your engine shakes at idle A car that shudders at a standstill but smooths out once you are moving is almost always losing combustion in one or…
• It hesitates when you ask for power A flat spot, stumble or brief hole when you open the throttle means the engine is not getting the air, fuel or spark…
• It failed the emissions test There are two quite different ways to fail: the car emitted too much, or the car could not prove it was testing…
• It stalls when you come to a stop Stalling as you lift off and the revs drop to idle means the engine cannot sustain itself at its lowest operating…
• The temperature is not where it should be Both directions are faults, and they are not equally urgent. Running too cold costs fuel and will eventually fail an…

Frequently Asked Questions