Ffs?

Hopefully someone here can help. In my digital fuel gauge FFS shows up sometimes and the check engine light is on. It is also really gutless.
I know the FFS stands for Feedback Fuel System. Any idea on what it could be? We inherited this 1990 Cougar LS, but unfortunally the previous owner was not good on maintenance.
Thanks!
Scott

Ffs basically means that the car is trying to calibrate the instrument cluster for the economy. Right now my car has a problem with the average economy setting. It counts all the way up to 99 and then stays there, which inturn is throwing off my distance to empty reading. I have no idea what my problem is, but am wondering about a faulty sensor.