The single core G5's used one type of air deflector, the dual core from 2005 used a different deflector. You can tell which it is by looking at the deflector area over the ram slots. The older model has a big dent there to fill the empty space over the fans. The dual core version is flat in that area, as it has a different fan duct design.
The switch that tells the G5 the deflector is in place, is inside the bottom rear slot. If you look at the plastic tabs at the bottom of the air deflector, the one in the back has a solid plastic sticker or paint on it. Thats for the switch. If that tab is somehow broken off, the G5 will think the air deflector isnt installed and the fans will switch to high speed.