Most header kits will require this because the primary (front) O2 sensor is mounted further down the header rather than right next to the head like it is mounted in the factory rear manifold. All of our o2 extensions are 27" long.
A. '97-05 Grand Prix/Regal/'00+ Monte/Impala/Bonneville (front), '04-07 Cobalt/Redline 2.0L
B. '04-07 Grand Prix (rear), '06+ Cobalt 2.4L, '08+ HHR/Cobalt SS (rear)
C. '06+ Grand Prix (front), '06+ HHR 2.2/2.4L (front)
D. 08 Grand Prix (rear)
Please verify the style of O2 plug you will need by looking at your actual plug on the car. Some early model cars have used the previous model O2 plug.
The 2004+ Grand Prix rear O2 is mounted far forward, so when you install a ZZP downpipe you will need to pick up the "B"('04-07) & "C"('08) o2 extension so you can use the factory rear O2 sensor.