Smog Repair Technician Practice Exam

When analyzing a vehicle showing high levels of hydrocarbons (HC) and carbon monoxide (CO), a Long Term Fuel Trim (LTFT) reading of -15% can provide key insights into the vehicle's performance, especially relating to the Positive Crankcase Ventilation (PCV) system. A negative LTFT reading indicates that the engine control unit (ECU) is compensating for a richer fuel mixture – in this case, reducing the amount of fuel being injected.

A LTFT of -15% suggests that the engine is receiving more air and therefore adjusting fuel delivery downward. This signals that the air-fuel mixture is imbalanced, and it's trying to correct that by injecting less fuel to avoid excessive richness. If the PCV valve is suspected to be an issue, a stuck-closed PCV valve can contribute to a leaner condition by not allowing the proper flow and mixing of crankcase vapors. If the valve is closed when it should be open, it restricts airflow and leads to an imbalance of the air-fuel mixture, potentially resulting in higher emissions of HC and CO.

In this context, a stuck-closed PCV would correctly explain the negative LTFT reading because it prevents the proper ventilation of gases, causing the