>>No codes (single blink) and the only abnormality is a slightly fluctuation in idle speed (between 500 - 700 rpm).

Need someone to give me a clue of the probable causes and a sequence of tests I could perform to find them and get them fixed.
>>
No codes stored on led/sw is just Check Engine module.
Are you getting a single flash [ no codes stored ] on pin #8 ???
You want to see what is stored on the HFM-SFI module. [ pin 8 with a homemade led tool]
That will give you fuel/ignition malfunction faults.......is any