If you are going to replace them it is best to use the GM oem sensors.
There was a post a while back where the dealer used one aftermarket sensor with one gm sensor.
It did not work very good.
They had to go back in and replace the aftermarket wit a gm OEM sensor.
There is a lot of wiring problems on the sensors and problems with water gettin them to.
A tsb on sealing them up with silcone.
I would suggest you get a all data sub and go threw the repair flow charts test things out while you are in there.
You may not even need sensors.
http://www.alldata.com/products/diy/index.html
Are you getting any codes?
If so post back code no.
MT