Message CO

I have a 1998 eddie Bauer Expedition and recently my gas gauge does not read the gas mileage to empty but it reads a message "CO". When I turn on the ignition it briefly reads 36 miles to empty (which is wrong). What is this error message and what do I need to have checked? Yes, it is riding rough but I was told it was the #4 coil and this is going to be addressed this upcoming week but I need to know what this message is because being a woman I do not want to run out of gas; I'm paranoid about my gas mileage and even though I pretty much have it figured out how mych gas I put in and how far it's going to take me, I find security in my gas gauge.