P0171
System Too Lean (Bank 1)
The engine is running with too much air or too little fuel on Bank 1.
The ECM uses the O2 sensors and fuel trims to maintain a 14.7:1 air-fuel ratio. When long-term fuel trim exceeds approximately +25%, meaning the ECM is adding significant extra fuel to compensate for a lean condition, this code is set. This is one of the most common check engine light causes.
Symptoms
Common Causes
Vacuum leak
HIGHCracked intake hose, loose clamp, leaking intake gasket, or broken PCV valve allows unmetered air in.
Dirty or failing MAF sensor
HIGHUnderreporting airflow causes insufficient fuel delivery.
Weak fuel pump or clogged fuel filter
MEDIUMInsufficient fuel delivery to match air intake.
Leaking fuel injector(s)
LOWA stuck-closed or partially clogged injector delivers less fuel.
Step-by-Step Diagnosis
Check fuel trims
mediumMonitor short-term and long-term fuel trims at idle and 2500 RPM. LTFT above +10% is significant.
Inspect for vacuum leaks
mediumCheck all vacuum hoses, intake manifold gaskets, PCV valve, and brake booster hose.
Clean or replace MAF
easyClean with MAF cleaner. If fuel trims dont improve, replace.
Test fuel pressure
hardCheck fuel pressure at the rail. Low pressure points to pump or filter.
Commonly Seen With
P0171 is frequently found alongside these codes:
Related Codes
Frequently Asked Questions
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