who should i see for a good price and what could the problem be? on 1996 Toyota Tacoma

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what could it be? theres many things ive looked up and i dont want to spend thousands on a bunch of different things if they fix the wrong thing. also, check engine light is always on. but truck seems to run great! but when turned off and i get out it sometimes makes a humming sound for a little bit. (fault code P0125, catalyst:notready oxygen sensor:notready evap system:notready)
(1) Answer
Bring it to the dealer for diagnosis , then bring it to a well equipped independent shop for a second diag.and see if they come up with the same.If your looking at thousands to repair , then the price of two diagnosis to be more certain would be well worth it. Whomever you have do the repairs , hold them accountable , for their diagnosis.