As long as the duty cycle is changing roughly 2 times a second, the system is able to adjust the mixture around the ideal. Does not matter much if it's at 2V or 10V, although ideally it will be around half of battery voltage so there will be enough range on either side to correct the mixture under other operating conditions. Just make sure the reading is still switching both at idle and at about 2500RPM. If so, go get the car retested for emissions.

Also, yes, the A/F adjustment is VERY sensitive; it does not take but a small fraction of a turn to make a big difference in the duty cycle reading. Also, wait for it to settle before readjusting.

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Erich Loepke
2010 Ford Focus
Currently Benz-less