The best accuracy you can get from a typical automotive grade wide band sensor is ±0.1 AFR, and that only in a range of 14:1 to 18:1 or so--this is a limitation of automotive WB O² sensors which were designed to run an engine, not to be laboratory grade instruments. Though that is pretty damned accurate, ±0.7%. The vertical channel of most digital oscilloscopes are not that accurate.
The specs for the Bosch LSU4 series sensors state accuracy as ±1.5 AFR (gasoline) at 25:1, and ±3.0 AFR at 11.75:1. The good news is that it is a lot more accurate on the lean side of 14.7:1. Remember these are the sensor's specs, any inaccuracy in the controller and display circuitry must be added in as well, meaning ± 0.2 AFR is about as good as it will be at the HID (Human Interface Device)...
Other maker's wide band sensors have similar specs.
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq