Updated Last reviewed September 15, 2026By CarFaultIQ Editorial Team
Verify with official recall databases, service records, a mechanic inspection, and local market data before relying on this summary.
At a glance
Quick diagnostic answer
Fuel trims reveal the computer adding more fuel than design to keep the mixture right. When long-term trim climbs past its threshold, P0171 sets: the system is running lean.
SeverityModerateUrgencyUrgency not yet verifiedEvidenceSource notes available
OBD code
P0171
System/category
Fuel & Air Metering
Severity
Moderate
Structured severity term
Urgency
Urgency not yet verified
Can I drive?
Usually OK to drive short-term
Mild lean conditions are tolerable briefly, but sustained lean running risks misfires and converter damage — diagnose soon.
Repair/cost band
$$ · Moderate
Estimate band only — actual cost varies by region, parts availability and labor rates.
Vehicle applicability
Universal across makes; engines with aging rubber intake parts and high mileage are the classic background.
Current applicability note is free text
Computed rule output may be more cautious than stored content and is never used to soften safety-critical risks.
Short answer
P0171 means the engine is getting more air (or less fuel) than the computer expects on bank 1. It's one of the most common codes on earth, and unmetered air — a vacuum or intake leak — is the most common cause.
SeverityModerateDrive?Usually OK to drive short-termRepairDIY – Moderate
Key takeaways
It's one of the most common codes on earth, and unmetered air — a vacuum or intake leak — is the most common cause.
Most common causes: vacuum or intake boot leaks (unmetered air) and dirty or failing maf sensor under-reporting airflow.
First diagnostic step: read fuel trims at idle and at 2,500 rpm — high trims at idle that normalize with rpm point to a vacuum leak.
Can I still drive with this?
Usually OK to drive short-term
Mild lean conditions are tolerable briefly, but sustained lean running risks misfires and converter damage — diagnose soon.
SeverityModerate
Moderate
Diagnostic flow
What it meansFuel trims reveal the computer adding more fuel than design to keep…
Check this firstRead fuel trims at idle and at 2,500 rpm — high trims at idle that normalize with rpm point to a vacuum leak
Likely causeVacuum or intake boot leaks (unmetered air)
Can you drive?Usually OK to drive short-term
P0171 is a generic OBD-II code, which means it carries the same core definition on every make — what differs is which causes are most likely on your engine.
Work the diagnostic steps below in order. They are sequenced from cheapest-and-most-common to most expensive, which is exactly the order a fair shop should follow.
Structured DTC reference
What P0171 means
The engine is running with too much air or too little fuel
A trouble code identifies the circuit or monitored condition that failed its test. It does not, by itself, identify the failed part or prove the same cause on every vehicle.
Cause categories to discriminate
Air leak at the inlet manifold
Split air intake hose
Cracked exhaust manifold
Cracked inlet manifold
Dirty air flow sensor
Failing fuel pump
These are structured diagnostic hypotheses, not a parts-replacement list. Vehicle context, symptoms, operating conditions, and test results determine which remain supported.
Severity and drivability
The code alone is not enough to assign one universal severity or “safe to drive” answer. Stop driving for loss of braking or steering, severe overheating, oil-pressure loss, a fuel leak or fire risk, or any rapidly worsening safety symptom.
Fuel trims reveal the computer adding more fuel than design to keep the mixture right. When long-term trim climbs past its threshold, P0171 sets: the system is running lean.
Common symptoms
Check engine light
Rough or hunting idle, sometimes stalling at stops
Hesitation under light load
Hissing from the engine bay in larger leaks
Likely causes
Vacuum or intake boot leaks (unmetered air)
Dirty or failing MAF sensor under-reporting airflow
Weak fuel delivery — pump, filter, or regulator
Stuck-open PCV or purge valve
Exhaust leak near the upstream O2 sensor skewing readings
Diagnosis steps
Read fuel trims at idle and at 2,500 rpm — high trims at idle that normalize with rpm point to a vacuum leak
Smoke-test or carefully probe the intake tract for leaks
Clean the MAF with proper cleaner and re-check trims
Check fuel pressure against spec if trims stay high everywhere
Inspect PCV and purge valves last
What a mechanic should check
Trim behavior at idle versus cruise is the diagnostic shortcut — insist the shop reads trims before replacing parts.
Vehicle applicability
Universal across makes; engines with aging rubber intake parts and high mileage are the classic background.
Repair actions / cost band
$$ · Moderate
Estimate band only — actual cost varies by region, parts availability and labor rates.
P0171 — vehicles with official records naming this code
P0171 is named in NHTSA-published manufacturer communications or investigations for 131 vehicle families, including:
A code named in a manufacturer communication means the document refers to it for those vehicles. It is not a diagnosis of any car, and it does not mean the code is common on that vehicle.
Frequently asked questions
Can a dirty air filter cause P0171?
Rarely — a clogged filter usually causes rich codes, not lean ones. Unmetered air after the MAF is the classic cause.
Is P0171 expensive to fix?
Often not: many cases are a cracked hose or dirty MAF. Fuel-pump-level causes cost more, which is why trim-based diagnosis matters.
Can I just clear the code?
It will return until the cause is fixed, and sustained lean running can damage the catalytic converter.