ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Medium
A vehicle owner reports that their check engine light (CEL) illuminates intermittently, and the engine sometimes hesitates under hard acceleration. A scan tool reveals a pending Diagnostic Trouble Code (DTC) P0171 (System Too Lean, Bank 1). The technician inspects the engine and finds no obvious vacuum leaks. What component should the technician investigate next?
- AThe exhaust gas recirculation (EGR) valve.
- BThe mass air flow (MAF) sensor.
- CThe evaporative emission (EVAP) purge valve.
- DThe catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: B. The mass air flow (MAF) sensor.
DTC P0171 indicates a lean condition. While vacuum leaks are a common cause, a faulty MAF sensor that under-reports airflow can also cause the ECU to inject less fuel than needed, leading to a lean condition and hesitation under acceleration due to insufficient fuel.
Why the other options are wrong
- A. An EGR valve stuck open can cause a rough idle and lean condition, but P0171 typically points to air/fuel mixture issues more directly related to metered air or fuel delivery.
- C. A faulty EVAP purge valve, if stuck open, could cause a vacuum leak, but the question states no obvious vacuum leaks were found. If stuck closed, it would not cause a lean condition.
- D. A faulty catalytic converter would typically cause DTCs related to catalyst efficiency (e.g., P0420/P0430) and would not directly cause a lean condition leading to P0171.
DTC P0171 (System Too Lean)
DTC P0171 indicates that the engine control unit (ECU) is detecting a lean air-fuel mixture in Bank 1, meaning there's too much air or not enough fuel.
- Indicates System Too Lean, Bank 1.
- Common causes: vacuum leaks, low fuel pressure, faulty MAF sensor.
- Leads to ECU adding fuel (positive fuel trims) to compensate.
Memory trick: P0171: 'Lean' means 'Less' fuel, 'More' air.