P0171 · Generic
P0171 - System Too Lean (Bank 1)
The ECU has determined the air-fuel mixture on Bank 1 is running leaner (more air, less fuel) than it is able to correct for using fuel trim adjustments alone.
Sourced from our reviewed reference database — not AI-generated.
Drive recommendation
Safe to drive short-term if the engine is running reasonably smoothly, but have it looked at soon — a sustained lean condition can cause a misfire and, over time, damage the catalytic converter.
Most likely causes
Vacuum leak (intake boot, PCV hose, brake booster line)
Dirty or failing mass airflow sensor
Weak fuel pump or low fuel pressure
Clogged fuel injector(s)
Exhaust leak upstream of the oxygen sensor
Reported symptoms
- Rough or high idle
- Hesitation on acceleration
- Occasional misfire
- Check engine light
- Reduced power
- Surging at idle
Recommended diagnostic checks
- Check long-term and short-term fuel trims on a scan tool
- Smoke-test the intake system for vacuum leaks
- Inspect and clean the mass airflow sensor
- Check fuel pressure against spec
- Inspect for exhaust leaks near the upstream oxygen sensor
What not to replace yet
Cleaning or replacing the mass airflow sensor as a first step without smoke-testing for vacuum leaks first — a leaking intake boot or PCV hose is a far more common cause.
Frequently asked questions
What is the most common cause of P0171?
A vacuum leak — most often a cracked or loose intake boot, PCV hose, or brake booster line — is the single most common cause.
Is P0171 the same on every car?
The definition (Bank 1 running lean) is standardized across manufacturers, but the most likely cause varies by engine design, so always check the fuel trim numbers and inspect the intake before assuming a specific part is at fault.
Technical details
- Code
- P0171
- Make
- Generic
This information is for diagnostic guidance only and is not a substitute for a qualified technician's in-person inspection. Always confirm a cause with the recommended tests before replacing parts.
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