P0171 means System Too Lean (Bank 1). The engine computer has added as much fuel as it is allowed on bank 1 and the mixture still reads lean, so more air or less fuel is reaching the engine than it expects. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0171 means
The engine computer keeps adjusting fuel to hold the air-fuel mixture near the ideal ratio, using oxygen sensor feedback. Those adjustments are fuel trims: short-term trim reacts instantly and long-term trim learns over time. Positive numbers mean the computer is adding fuel; negative numbers mean it is taking fuel away. When long-term trim sits near the limit the computer allows, it sets a lean or rich code. On a V engine, bank 1 is the side containing cylinder 1 and bank 2 is the opposite side; inline engines only have bank 1.
P0171 symptoms
- Check engine light, often with no other symptom at first
- Rough or hunting idle
- Hesitation when accelerating
- Misfires on a lean bank, sometimes with P0300
Common causes of P0171
- Vacuum leak from a cracked hose, intake gasket or the PCV system
- Dirty or failing MAF sensor under-reporting airflow
- Low fuel pressure from a weak pump, clogged filter or faulty regulator
- Clogged or leaking fuel injectors
- Exhaust leak ahead of the upstream oxygen sensor, which lets air reach it
How to diagnose P0171
- Read freeze-frame data to see the engine speed and load when the code set
- Compare long-term fuel trims at idle and at 2,500 RPM: high at idle and better at speed points to a vacuum leak; high everywhere points to fuel delivery or the MAF
- Smoke-test the intake for leaks
- Clean or test the MAF sensor
- Check fuel pressure with a gauge against the specification
Can you drive with P0171?
Usually yes for a short time, but a lean engine can misfire and run hot, and a large leak worsens drivability. Fix it promptly, especially on turbocharged or supercharged engines.
P0171 tips
If P0171 sets mainly at idle and fuel trims fall toward normal at 2,500 RPM, a vacuum leak is the prime suspect, because a fixed leak is a bigger share of total airflow at idle. If trims stay high at every engine speed, look at fuel pressure and the MAF sensor instead. After an air filter change, make sure the intake boot after the MAF is clamped tight, since any air entering past the sensor is unmetered.
Will P0171 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0171 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0174 | System Too Lean (Bank 2) |
| P0172 | System Too Rich (Bank 1) |
| P0101 | Mass or Volume Air Flow Circuit Range/Performance |
| P0300 | Random/Multiple Cylinder Misfire Detected |
Guides that help
- fuel trim explained
- fuel filter
- engine sputtering
- vacuum leak
- MAF sensor symptoms and cleaning
- fuel pump, filter and regulator symptoms
- PCV valve
When it is more than a sensor
Fuel trim codes are almost always fixed with a leak repair, a sensor or a fuel delivery part. If a long-running lean condition has burned a valve or piston and a compression test confirms it, compare repair against a replacement on our engines page and see engine replacement cost.
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