Barometric pressure
Atmospheric pressure from the car's own sensor. The unit uses it to know how much oxygen the air holds: there is less of it in the mountains and the mixture is worked out differently. Hence the familiar «the car will not pull in the mountains» — not a fault but an honest recalculation of delivery. On turbocharged engines the same figure takes part in the boost calculation: PID 33 is the reference, while PID 0B and PID 87 show how far the intake pressure has risen above it. If the sensor lies, the engine runs lean or rich equally in every mode.
What is normal
About 100 kPa at sea level. A much lower reading without mountains means the sensor lies, and the mixture is calculated from a wrong baseline.
How it feels behind the wheel
The same fault shows itself in different ways. Each sign below says what to look at in the live data.
How to see it
In CarScan: the Live data tab, «Start polling». The app asks the car several times per second, draws a graph and remembers the highest and lowest value of the trip.