mine seems to have lost power so i was wondering which fuse to check.

 

Which year/model?
Also which engine, I doubt that matters but just in case.

 

2003 e350 7.3l passenger extended

I need to remember that.

 

Ok so until around 2005 wiring diagrams were not great but its what weve got. There are a total of 5 power ports shown, I took a snip of two different pages overlapped so all 5 are in one picture. I think we can safety ignore the one labeled front. The other four are not labeled for location though. to determine which are which I would first locate all of them in the vehicle then remove fuse 28 and see which port no longer works and cross out that one from the diagram, reinstall fuse 28 and repeat the process with fuses 33, 37, and 39 one at a time.
Once you narrow down which one is the one that doesnt work in your vehicle we can go from there, please use the connector number to identify which lighter, for example the front one would be C2031 as that is the connector number for the lighter.