The O2 sensor fine tunes the mixture once the engine's hot. The duty cycle sets a starting point and needs to be adjusted correctly when the engine is hot.

So, invest in the meter. I know I have.

As you say, the sensor should be replaced as a preventive measure. Mine's over 7 years old so a new, MB original one's on the way as I write. I hope to be able to report better town fuel mileage once it's installed and I've set the duty cycle.

RayH