[jmb99]

> O2 sensor in cars compare the O2 concentration between the outside air and exhaust gases, it needs outside air as a reference

Source?

[GuB-42]

https://en.wikipedia.org/wiki/Oxygen_sensor#Automotive_appli...

It looks that some O2 sensors that don't require a reference have been used (titania sensors) but even though they have some advantages, they are less precise and mostly obsolete.