The 2021 Honda Civic setting P0171 indicates the engine control module detected a lean condition on bank 1—too much air relative to fuel. The 10th-generation Civic's turbocharged 1.5-liter engine requires systematic diagnosis to identify the lean source.
Understanding P0171
P0171 indicates long-term fuel trim exceeds approximately +25%, meaning the ECM consistently adds fuel to compensate for detected lean conditions. This points to excess air entering the intake or insufficient fuel delivery.
Civic Turbo System Considerations
The 2021 Civic uses a turbocharged 1.5-liter four-cylinder in most variants. The boost system introduces additional potential leak points compared to naturally aspirated engines. Intercooler hoses, charge pipes, and connections can develop leaks affecting fuel mixture.
Vacuum Leak Detection
Vacuum leaks allow unmetered air into the intake manifold after the mass airflow sensor. Common locations include intake manifold gaskets, PCV system hoses, brake booster line, and throttle body gasket. Smoke testing effectively reveals leaks.
MAF Sensor Issues
A contaminated or failing MAF sensor underreports airflow, causing lean conditions. Clean the sensor with MAF-specific cleaner. If cleaning doesn't help, replacement may be necessary. Aftermarket air filters sometimes contaminate MAF sensors.
Fuel System Testing
Verify fuel pressure at the rail meets specification. Low pressure from a weak pump or restricted filter prevents adequate fueling. Check fuel injector operation and spray patterns.
Repair Costs
Vacuum hose replacement costs $50-200. MAF sensor replacement runs $150-300. Fuel pump replacement ranges $500-900. Boost hose replacement costs $150-400.