The probe has attenuation built in... I think it's 10x. Arduino is grounded to the vehicle chassis. The ECM is also grounded to the vehicle chassis but I don't know if it has any isolation built in. The scope... I haven't a clue. I believe it automatically centers itself and probably it deletes any DC component.
Maybe I should amplify the signal and put a 5v zener in front of it. Problem is, any op amp is going to need a split power supply which is a PITA just to double the voltage.