Hello, 

My toyota ciena 2001 is consuming too much fuel and i discovered the catalyst has been stolen and after i bought another one and fix its still consuming fuel..... Someone say i should change the oxygen sensor..... Can some here help with what to do please.

You need to seek an expert advice

If very simply first check on ur dashboard n check if check light is on, then bath if is on  check the 

Your spark plugs are responsible for sparking combustion in your engine. If they misfire, or are working poorly, this can affect your gas mileage in a negative way. Having bad fuel injectors can cause bad mileage. ... This causes more fuel to be used, and will use up more gas than you need. Air filter can cause it, check if ur air filter is dirty, then change  or clean it and check all ur  catalyst oxygen sensors   and know the one that is leading and the one that is not leading , check the one that is not leading