P0171
P0171: System Too Lean (Bank 1)
What P0171 means
P0171 is “System Too Lean (Bank 1).” Your engine control unit adjusts the fuel mixture constantly based on what the oxygen sensor reports. When the exhaust keeps showing more oxygen than it should, the ECU adds fuel to compensate. P0171 is the code it sets when it has added as much as it can and the mixture is still lean.
“Bank 1” is the side of the engine containing cylinder number one. On a four-cylinder there is only one bank, so the distinction only matters on V6 and V8 engines. If you also see P0174, that is the same fault on Bank 2, which usually points at a cause affecting both sides equally.
The important thing to understand: P0171 tells you the symptom, not the cause. Something is either letting in unmeasured air or failing to deliver enough fuel.
Common causes
Roughly in order of how often they turn out to be the culprit:
- A vacuum or intake leak. Air entering after the mass airflow sensor is air the computer never counted. Cracked intake boots, brittle vacuum hoses, and failed gaskets are the classic offenders.
- A dirty mass airflow (MAF) sensor. If it under-reports airflow, the ECU under-delivers fuel. This is the cheapest thing on the list to address and worth ruling out early.
- A weak fuel pump or clogged filter, delivering less fuel than commanded.
- A leaking or restricted fuel injector.
- An exhaust leak near the manifold, which lets ambient air reach the upstream oxygen sensor and makes it report a lean condition that isn’t really happening in the cylinder.
DIY-fixable, or shop repair?
Partly. Cleaning a MAF sensor is genuinely a beginner job: a can of MAF cleaner, a screwdriver, and a few minutes. Because it is both cheap and a common cause, it is the sensible first move.
Finding a vacuum leak is harder. Sometimes you can hear it, or find a cracked hose by inspection. Often you cannot, and a shop’s smoke test is what finally locates it. Fuel pump and injector work is shop territory for most people.
Typical cost range
Sources put the range at roughly $50 to $900, and that spread is real rather than vague. MAF cleaner is under $10. A new air filter is under $50. A fuel pump or injector replacement runs into several hundred dollars or more.
This is why the diagnostic order matters: the difference between the cheap end and the expensive end of that range is which cause you have, and the cheapest causes are also among the most common.
Should you keep driving?
A lean condition is not an immediate roadside emergency, but it is not harmless either. Running lean raises combustion temperatures, and sustained lean running can damage pistons and valves over time. It will also cost you fuel economy and fail an emissions test. Treat it as something to diagnose in the next week or two rather than something to ignore for a year.