There are two types of guage....

WIDEBAND
Normally has a 5 wires or more for the sensor and will show accurately at all
times the correct AFR .
Not cheap but worth the extra expense for the confidence in the engine.


NARROW BAND
This is only accurate around the stoichometric ratio (perfect combustion point)
Turbo cars need rich mixtures on boost, a narrow band sensor can only say
lean, normal or rich but not how rich.
Although some of these on the market do show an attempted AFR but this
is highly inaccurate and unstable.
This is a cheap method but far fom ideal. More suited for non turbo cars.