My 89 Cougar Fuel gauge reads CO instead of miles/gallons to empty. If anyone can point me in the right direction, it would be appreciated

so i just recently bought a 1970 cougar standard hardtop. it's a project i've been working on for only a few weeks, but things are coming along nicely. i'm fairly new to classic cars though, and the fuel gauge isn't working, so i was just wondering what could be causing this? i'm not looking...