Check engine light in for months then go off for an hour then comes back on

My check engine light came on so I replaced
the O2 sensor and the gas cap. It went off for
a couple of weeks, now it's back on again.
The error code says it is the O2 sensor again.
I did just replace the fuel pump and I'm not
sure it sealed properly on the top of the tank.
Could that cause an O2 error? Also I should
add that when I fill up, I smell gas inside the
truck for the first hour or so.