Check engine light on

My check engine light came on my jetta so i took it to my mechanic and he put the code machine on the car and he said it was the throttle body. So i bought one and he replaced it. Car works fine but after a few hours on the road the check engine light came back on and the gas cap light came on. The gas cap light turned off but thr check engine light is till on. Does anybody know or have any suggestions why?

Jetta is running fine

If it is running fine,may just be from the gas cap issue,reset the ECM and should turn light off.

Ok now the RPM go up when im stopped. It goes up to like 13 rpms. Can there be an air leak or a bad sensor since the check engine light is still on?