On the back of the inlet manifold, there is a brown plug - that connects to the AAC valve.
However, it's more likely to be a mis-aligned throttle position switch (TPS) (black box sitting on the side of the throttle body). It has a switch inside that tells the ECU that the throttle is closed. Sometimes it gets out of alignment and doesn't make the connection, so the ECU thinks the throttle is partly open and adds more fuel.
To test it, you need a multi-meter. Remove the plug, and measure the resistance across the top and middle pins. It should be 0. If not, loosen the 2 retaining screws and gently rotate the TPS until you get 0 ohms, then re-tighten the screws. (it often makes the job a bit easier if you crack the throttle open with a 12 thou / 0.3mm feeler)