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.