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Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.The code P2138 is for the Accelerator Pedal Position (APP) sensor when the outputs are dissimilar and not usually caused by a failing Throttle body and or any relays and in most cases the fix to repair this code is to replace the Accelerator Pedal Position (APP) Sensor. This is a very common failure as well on the Outback as well. I have enclose the steps in the link below to test the APP as well. Copy and paste the link to your browser to open and view them.https://dl.dropboxusercontent.com/u/104923897/APP%20test%20Jan%2017.pdfSee the location in the link below as well.https://dl.dropboxusercontent.com/u/104923897/APP%20location%20Jan%2017.pdf
There are no relays in the system for this code so I do not know what relay they are referring to. If replacing the pedal and sensor solved the problem you should be good to go.