The 2009 Hyundai Sonata has two unique oxygen sensors. One is known as the upstream oxygen sensor,and the other is known as the downstream oxygen sensor. They each have unmistakable purposes,so You'll have to realize which You're after before You begin eliminating or supplanting. The upstream O2 sensor is situated on the ventilation system,which You'll have the option to find by following Your exhaust pipe towards the front of the vehicle. The downstream sensor screws into the tailpipe,just after the exhaust system. Evacuation or substitution will require raising the vehicle and putting it on jack stands,and a lot of power; since the exhaust can get very hot,anticipate that an '09's sensors should be warmed and cooled into place very well.