2007 Yaris with P0138 code

[mnbvczxc]

I don't know if there is a sub-code for this but if there is then it's 07E8, it's just that I'm using a buddy's OBD unit and I know almost nothing about them or these cars for that matter. My question is what part would you replace first and where abouts is it on my car. I'm good with a wrench so you just have to point me at the defective part . . .

Thanks for any and all help.

Ron