"Service Engine Soon" ON but car seems OK

[DSMvroom]

Hi guys, my car's "service engine soon" light just came on but the car seems to be running fine. I used the search button and someone said to go to autozone and get the codes?? Do they charge for that? And will you guys be able to tell me wtf is wrong with my car from the codes? It doesnt seem like anything is wrong with it. Thx for help guys.

FYI : it is a 99 GST with 64.7k miles

 

[Tech9Talon]

I have the same problem. When I ran the code it says P0120 TPS Circuit Malfunction. I'm just trying to find out how we're getting to an o2 sensor problem from this code? I'm going to try to clean the terminals and hope that works, but if not I saw a few TPS's in the classified section of this site for $25. Local auto parts store wanted $200+.