Mmmm, it sounds like the calibration is off - but you cannot reasonably expect to get a decent free air calibration from a used sensor.
You might benefit from pulling the sensor from the exhaust, turn the heater on, and leaving it to burn off any soot and contamination.
I've been lucky enough to find people with 6gas testers that allow me to mess around. Knowing Lambda is only part of the story, it would be handy to know your HC and CO to make sure that your injectors are indexed with the engine correctly, strange things can happen with part burned charges.