The oxygen sensors are used by the computer to calculate the air/fuel ratio for the engine. A failed or malfunctioning O2 sensor will definitely cause fuel consumption issues!
For reference, my V6 rondo can get 8L/100km if driven gently on the highway, and around 12L/100km in town.

 

I've replaced the oxygen sensor and the consumption is lower than before now it's 16l per 100 km it is still high but it's better then before.

 

I just bought a KIA RONDO model 2012 EX 2.4L with 196000 km on the odometer.
I did some consumption tests and found 18litres per KM which is enormous for such an engine.
Most of my journeys are in town.
I've got the check engine light on and the mechanic told me it's the oxygen sensor that needs replacing, is that the cause?
Is this normal consumption? Is there a solution?

Did you mean 18 L per 100 km ? - that would be 13 mpg which is heavy but a little heavy footed and mostly town driving is quite likely for a heavy vehicle.

Get the stored fault codes read - if the O2 sensor needs replacement then there will be a code.

 

Thank you so much for your swift response
I'll change it this week end and I'll check and give you a feed back