P0171 – System Too Lean (Bank 1) means the engine computer is adding far more fuel than expected to keep the mixture correct. Either unmetered air is sneaking in, or fuel delivery is weak.
Start with the $20 fix
The most common cause is a dirty mass airflow (MAF) sensor. The MAF measures incoming air; when its delicate wires get coated in dirt and oil film, it under-reports airflow and the computer runs the engine lean. A can of MAF cleaner costs about $10–20:
- Unplug and remove the MAF sensor (usually two screws on the intake tube).
- Spray the sensing wires generously with MAF cleaner – never touch them.
- Let it dry fully, reinstall, clear the code, and drive.
If the code returns
- Vacuum leaks: cracked intake boots, brittle hoses, and failed intake manifold gaskets let unmetered air in. Idle RPM that fluctuates or hissing sounds are strong hints.
- PCV valve and hose ($10–25): a stuck-open PCV system is a hidden vacuum leak.
- Weak fuel delivery: clogged fuel filter or tired fuel pump – more likely if the car hesitates under load.
- Exhaust leak before the O2 sensor: skews the reading lean.
Confirm with live data
With a live-data OBD2 scanner, watch long-term fuel trim (LTFT). Healthy is roughly ±5%. If LTFT is +15% or more at idle but drops near zero at 2,500 RPM, that's a vacuum leak signature. High trims at all RPM point to the MAF or fuel delivery.
Watch the full P0171 walkthrough on our YouTube channel, and check scanner compatibility for your car.