volk90s

it failed long time ago and I never had a chance to fix it. Usually when I put full tank of fuel it goes up and stays there for some time and then goes down and then sometimes up again. It is very intermittent. What is it? What are the costs involved to fix?

egp320i

its probably shorting out or just crapping out.
depending on car its going to be of course attached to the fuel pump which might have to be pulled to be replaced.

CvTech

My new, actually used, cluster seemed to fix the problem. I think, for me at least, there was some short in the panel or even the gauge itself that this would happen to me.

My fuel gauge, when full, would go all the way to F. Then sometimes it would drop almost to E, then go back up like to 1/4 tank. Very intermittent readings.

I suggest finding a cluster, pluging it in and diagnosing it that way. If the new/used cluster gives solid reading...then you knwo its not the fuel tank sensor, to which I think is kinda hard and difficult to repair or replace.

SCarGo02

If you have factory tools for doing the fuel pump, it'll help speed up the process, but not necessary IIRC.