There's no "voltage" conversion. The ECU outputs data that is already formatted. That's
the OBD2 specification. All sensors that are supported by the ECU will be in a measured
quantity format. In other words, the RDD or a scan tool will ask the ECU what the
sensor reading is at that time and the ECU will respond. For example, the RDD will ask
for the IAT (intake air temperature) and the ECU will respond with a data byte 0 - 255
which is the temperature in Celcius - 40 degrees. So, the range of temperatures is
-40C to 215C. All units are in metric. The RDD converts for English mode. Manifold
pressure range is from 0 - 255 kPa. So, yes it will read positive manifold pressure. In
screen mode 6, the RDD shows RPM and MAP (if a vehicle is equipped with a MAP sensor)
and a conversion to psi if the pressure goes over 100 kPa (positive pressure). A local
tuner shop made some suggestions as to what data to show and how to show it. So, at
this time the MAP is only displayed in kPa. The MAF is displayed in g/s. The o2 sensor
can display voltage or A/F ratio if it is equipped with a wideband sensor. When the RDD
initializes, it asks the ECU what sensor data it supports. So if a vehicle has a MAP or MAF
or wideband o2 sensor, it knows. As far as accuracy, the RDD simply displays the values
it gets from the ECU, so the values are only as accurate as the ECU's interpretation. But
since the ECU was engineered to meet some pretty strict emissions standards, I would
venture to say that the values are extremely accurate.