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So I just got a 75mm throttle body and plenum and I installed it. After I installed it I started it and the rpms went straight to 2,000. After about 5 minutes of looking at it I turned the AC on and it went down to 1,500. So I went out and got a new gasket for the IAC and the throttle body plenum to intake manifold gasket. I also went out and bought a new TPS for good measure. None of these things solved the problem. At that point I bit the bullet and took it to a shop for them to look for exhaust leaks in case I was missing one. They gave it back to me saying there were no exhaust leaks and they couldn't figure out why it was idling high. I have an SCT Tuner and had already tried setting the idle rpm to around 600 and nothing changed. It drives great and I can feel the throttle reponse and high end pick up faster. After about 3 or 4 days of driving like this the service engine soon light comes on. So I take it and get the codes read and it's giving me the P1121 code. It still runs great except for the high idle rpms. Can anyone help me? I have no idea what's wrong anymore.
 
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