If you suspect that the closed loop system is not properly controlling fuel delivery, one of the first checks you should perform is an Oxygen (02) Sensor signal check. Since the ECM relies on the 02S signal to fine tune injection duration during closed loop operation, an accurate check of the 02S signal is crucial in diagnosing problems that you suspect are the result of improper closed loop control.
Remember, the engine (and engine control system) must meet certain conditions prior to checking the 02S signal or your results may be inaccurate. This usually means that the engine and 02 sensor must reach operating temperature, the feedback system is in closed loop, and engine speed is maintained at a specified rpm. 02S signal checks can be performed on OBD/OBD-II vehicles by using the Diagnostic Tester. Older vehicles may require you to backprobe the 02S signal wire using the Autoprobe or digital multimeter.
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