MIL Question - Voltage read by ECU keeping car in open loop?

I'm putting an O/R X on my new cobra and just spoke to the guy that does my exhaust work about MILs. He said that they aren't the greatest thing in terms of solving the CEL problem, and after looking into it a bit I have a question regarding the typical MILs that we're getting (Steeda, Dallas, etc.).

Are these plug in MILs just voltage clamps that throw back 0.45V to the ECU or do they have circuits in them to vary the voltage read by the computer?

What I've been reading about the function of the rear O2 sensors in relation to A/F is that when a 0.45V is read constantly read by the computer the computer interprets this as meaning the sensors are too cold to be used and this makes the computer stay in open loop, or warm up mode, and keeps the AF a bit rich. While this keeps the CEL light off, it ain't great for performance because you're always going to be a bit rich. I've searched the net for some specs on the MILs, but can't find the info regarding voltage variation, anybody got an answer??

COBRA90GT said:

Your guy who does exhaust work is full of BS. MIL Elims (when properly hooked up, as with any aftermarket device) work just fine - it doesn't matter if the rear O2 voltage is "constant." Your front O2s are the ones that affect drivability anyways - the rear O2s are strictly there for monitoring purposes.

I use the ones made by PEP, www.ponyexpressperformance.com . There's a great FAQ located at the aforementioned URL as well, might want to read up on MIL Elims on the PEP website.

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Missouri_guy answered the question.

Yes, the rear O2s aren't actively used in A/F adjustment, all they monitor whether the cats are working, but if the MILs supply a constant voltage, telling the ECU that the rear O2s are too cold to be read then the ECU will just keep the car in warm up mode until its told that the sensors are warm and if they don't it will eventually throw a slow code. I just talked to the SVT tech at my dealership and they had a guy with an 03 cobra come in with malunctioning rear O2s and what they found was that the sensors weren't toggling enough so the car had gotten stuck in warm up mode.