mazda 626 cel problem

[slimmyslim1420]

Ok I have a 98 626 2.0 4cyl. The check engine light is basically always on and I've done numerous things to get rid of it. The most recent thing I did was replace one of the o2 sensors which showed to be bad on a diag test. After about a week the light came back on. I ran another test and came up with catalyst sytem below threshold. Which has a pretty broad range of possibilities. Now here is the weird part. After paying close attention to the light for about a month, it goes off when the gas tank is almost empty. And comes back on if you fill it up above a 1/4 tank. If you don't put more than a 1/4 tank it stays off. But it always goes back off once the tank is almost empty. I open to all suggestions, all help is appreciated. If you need to know anything else don't hesitate to ask.
thanks