check engine light stays on replaced oxigensencers and spark pluges ?

They ran smoke test and no leaks. Gas mileage poor running rich.Black dust in exhaust pipe.I use BP gas.Should I switch gas? Should I unplug battery and wait 5 min. then plug in?

what about the fuel pressure?? what kind of o2 sensors?? are they switching fast enough?? do you have a scanner?? forget about the battery. whoever is looking at your car will need the freeze frame data to see the data stream when the fault occured. Roy