Toyota 4Runner Service Light Coming on

My 1999 4Runner threw two codes when the engine check light came on. Toyota of Greenville checked the codes and found it was the catalytic converter and also the throttle body. They fixed the throtle body and reset the codes and I took it to a muffler shop. They checked the catalytic converter, which they replaced last year, and said it was just getting old and it looked good, but will always and forever make the check engine light come on. It did. I took it back to the Toyota place and they ran the codes again and again it was the new throtle body- which they said they cleaned out. The light went off after they reset it and now the light is on again!! My car is also eating up a LOT of gas but feels and runs smoothly. What should I do?

Thanks, Alli