Every Geo built after 1980 includes an oxygen sensor.  The Geo
oxygen sensor is part of its emissions control system.  It is
there to help your engine perform at its optimum level while
keeping emissions low.  Your Geo has a unique oxygen to fuel
ratio that is considered perfect.  The oxygen sensor can read
that ratio and send the information to your engine's computer. 
Too little air in the mixture is considered rich, while too much
air is considered lean.  Either way, the information is read by
the oxygen sensor and sent to the engine's computer.  The
engine's computer will then adjust the fuel level until the
ratio is back to its optimum level.  A broken oxygen sensor will
not be able to send this vital information to the engine's
computer.  This means that the engine's computer has to "guess"
the air level.  This may result in hesitation, poor
acceleration, surging, emissions test failure, and poor engine
performance.