are you sure there is a scan button ?
Reason I ask is I have had several Sirius receivers (external, not built into the dash) that do not scan channels. They normally have station presets that you can set to you liking and a jump button (goes to your favorite) channel. But I've never seen a scan function. Normally you have to manually turn the knob or button on the unit itself to scroll through channels then you select the one you want.