O2 sensors fuse in engine compartment box?

I am having a crank with no start problem. I've checked everything I can possibly think of fuel at rail is 49PSI, spark is present when cranking, etc. So, I went to the ASD and Fuel pump relays to check them, and relays are in working order, but I also checked fuses in that box and every fuse has power on both sides except the O2 sensors. I pulled the box and inspected wires on the underside, and all looked fine. Is this fuse fed as constant power, or where does it get power from?