2007 Pontiac G6 Impact sensors

I have recently bought a salvage title 2007 Pontiac G6 and have fixed it up. I installed two new front impact sensors and then I brought it into the dealership with a new air bag module and the new air bag so that they could program the module for me and put the air bag in. After finishing they told me that I needed new seat belt pretensioners. So I bought new pretensioners and installed them, but the car's air bag lights still came on. I brought it into the dealership to erase the codes and they told me that all the front and side impact sensors are bad, but the last time it was in the shop they said they were all good and I had already replaced the two front impact sensors. What could be the problem with the car?
Thank You.