[sokoloff]

EGR back EMF is simulated with a resistor bridge (for the "EGR not engaged" signal) and a diode to alter the effective resistor bridge (for the "EGR engaged").

Agree that it's totally counterproductive to remove an upstream O2 sensor for a road application. (For an off-road application, the only reason is to allow the use of TEL (leaded) fuel additive as an octane booster and an open loop controller and any such application would drive so few miles per year that it doesn't need to be worried about. The reason to eliminate a downstream is to remove (or dramatically reduce) the catalyst function. I have not experienced a car that did a "compare downstream to upstream"; I'm sure they exist, but manufacturers have an incentive to reduce nuisance MILs, so most controllers are designed to detect gross defects and typical failure modes of installed components, assuming all specified parts are installed, not to detect all possible types of intentional tampering.

I agree with you that this isn't a significant issue in terms of pollution levels, as the enthusiast market that will mod their cars to this extent isn't large in numbers, but based on the amount of poorly modified cars I see (gassers with tailpipes so sooted up they look like diesels, diesels modified so they can better "roll coal", people who think "more fuel must be better" when changing ECU maps, etc), I wonder how many regular cars' emissions are equally by some of these poorly modded enthusiast cars.

(I love and have no philosophical objection to auto enthusiasts and modded cars. Despite my hands-on experience above, I now drive an electric LEAF, which is not exactly an enthusiast car, but does payback the environment for some of my youthful transgressions...)