Hey guys! i hope youre all well? I need some help regarding mileage and sensor issue. My car displays 'Engine Malfunction' light in the display and after getting checked from Khan Motors in karachi i found out that the Air-Fuel Sensor of the vehicle is not working and need to be replaced. The cost of 2 sensors he quoted was 35k. I am not sure if thats the issue or the cost is actually that high. Please guide me what i should be doing. Current Average on 75%Petrol and 25%HiOct in city with AC is 43MPG that is 18.28 KM/L. Not sure if its good enough or Vitz usually give better average than that.

Will be really thankful if you guys help me out. This is urgent. Thanks.
Just incase you havent seen my car yet please dont forget to check it out. 
https://www.pakwheels.com/forums/members-member-rides/177783-my-new-vitz-manual-transmission-2007-a