I build test fixtures for a living, with multiple LED's.
LEDs have one lead longer than the other. This is the anode. This longer lead must be connected to the (+), and the shorter lead (cathode) must be connected to return, or NEG.
I don't know how an LED would effect a PCM, but there's always a resistor inline with the cathode... say 1K Ohm.
I know when I did my swap I read a lot about the ODBII connection to the ECM needing a resistor. I used the factory harness and ECM so I didn't worry about it. It works fine that way. But, my memory could be failing and I may have added one. I'd have to look at my notes.
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