The sensors used in automotive systems
are of a type called zirconia sensors. At high temperatures (like those in
the exhaust gas stream) they act as oxygen electrodes, with a very high
sensitivity and quick response (milliseconds). The sensors are used in a
feedback loop to control the air/fuel mix, partly to enhance engine efficiency,
and partly to meet current smog regulations.
At LOP mixes, they sensor does see a
small amount of free oxygen in the exhaust gas stream -- otherwise the whole
scheme wouldn't work.
As others have pointed out, the
sensors, unfortunately, are "poisoned" by lead in gasoline, so they would be of
little use in engines burning 100LL. Too bad, because otherwise it would
be a pretty simple matter to provide automatic mixture control.
Jim Cameron
(Waiting for a Legacy)