You can’t just swap parts on the throttle system. A relearn procedure needs to be completed after any work. Sometimes it’s a manual procedure, sometimes it’s done through the diagnostic tool. I would ask the mechanic if he did this.
Any number of things could cause poor gas mileage. It could be spark plugs, coils, dirty air filter, anything really.
Best thing to do is do a full engine scan for fault codes and repair as necessary. Check condition and functionality of all mechanical parts not included in the scan. Then you can start looking at live engine data while driving to see if/what the problem is.
Another issue, if a TPS has been incorrectly fitted, ie at the wrong angle, it can cause shifting issues. The ECU isn’t getting the correct information so can’t produce the correct output. It thinks you’re on half throttle when you’re on full throttle so shifts gears early.
I recommend looking for a good technician who has experience outside of doing oil changes and changing brake pads.
Recently i got my Throttle Sensor replaced with a second hand part after the car had issues with shifting up while on drive. After the repair the issue was fixed but i am noticing that i am getting poor mileage i.e. below 10kmpl before and 11-12kmpl now. Could it be a faulty sensor?
It's good manners to turn your car off when refilling. The fuel boy is standing outside all day in the heat and standing beside a hot running car isn't nice for anyone is this weather.
Anyway, some cars won't run properly without the fuel cap because of the way the EVAP system is set up. Might be normal for your model. Still worth finding a friendly mechanic and asking him to run a scan and see if P0440 comes up then you can investigate further.