A Technician Is Testing a Fuel Injection System with a Scan

A technician is testing a fuel injection system with a scan tool. While the engine is running in closed loop, he loosens a vacuum hose to create a small vacuum leak. Initially the STFT rises and the LTFT remains unchanged. After a few minutes the STFT returns to near zero and the LTFT rises. This test proves that ______.

A) the system is running rich
B) the system is running lean
C) the O₂S sensor is faulty
D) the system is working normally

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