1998 Jeep Cherokee co2 sensor

Where are the co2 sensor located

updated 2-10-10
thank you I know theres one on the exhaust but im fixing this jeep for a friend and she said the gargage was going to charge 1000 dollars to fix it, cant imagine the only one I seen on the exhaust costing this much is there another one on the this 98 jeep grand cherokee. Shes said the engine light is on and they said its coming back oygen senor thank you for help. Bob