What does a P0171 code mean?
What the computer is seeing
The engine computer adjusts fuel injector pulses based on oxygen sensor readings and airflow. When it keeps adding fuel to compensate and still cannot reach the target mixture, it sets P0171. The code points to a mixture problem, not to one specific part that is broken.
On engines with more than one bank, bank 1 usually refers to the side with cylinder number one. Check your owner's manual or a repair database to confirm the layout for your engine.
Usual suspects to check
Vacuum leaks are the most frequent cause. Cracked hoses, a loose intake boot, or a leaking PCV hose let unmetered air into the engine. A dirty or failing mass airflow sensor can also report the wrong airflow, and a weak fuel pump or clogged fuel filter can starve the engine of fuel.
- Vacuum hoses and intake boots
- Mass airflow sensor cleanliness
- Fuel pressure and fuel filter condition
- Oxygen sensor readings in live data
Narrowing it down
Read freeze frame data to see the conditions when the code set, such as idle, highway speed, or cold start. Live data can show short-term and long-term fuel trim values. Large positive trims often point toward unmetered air, while a fuel delivery problem may show up under load.
Common mistakes
- Replacing an oxygen sensor first without checking for vacuum leaks that cause the same code.
- Clearing the code and driving away without knowing what set it.
