2001 Toyota Echo Oxygen sensor

Engine starts running rough. Air filter container vibrating creating noise. Engine appears to want to stall. "Check Engine" light is illuminated on dash. Upon inspection a hose is not connected to air filter assembly. Reattach hose but problem persists. Now I also smell gas. Air filter is clean.
Where is the oxygen sensor?
Is the oxygen sensor the problem? If not the sensor, what are possible alternative problems?
How much does it cost to fix?
Maybe detached hose is supposed to be detached and is unrelated to problem.
Disconnected pos. Battery cable for 3 min. To erase code due to detached hose but problem persists.