failed smog test do to cat, & gas cap. Gas cap seems to be in good condition, &

The mechanic said he can't get a code, the readout says cat-fuel. So he told me to drive it for 100 miles n it should pass, done it, still won't pass, he says drive it another 50 miles. Still get the same readout, no code. The car purrs like a citten

battery been unhooked recently?

Yes, I unhooked the battery to reset the check engine light

anytime you unhook the battery,the computer resets all monitors to not ready.then you have to do what is called a drive cycle to set all monitors to ready.this should be done over a few days of normal driving,around town,stop and go,interstate,etc.the monitors if no problems should reset to ready after 50-250 miles.why was the check engine light on,and did you fix the problem for that light?and why did you unhook the battery?because if you fixed the problem,then the light would have went out on its own,or you could have had it cleared,instead of unhooking the battery and resetting everything?