The capacitor ignition coil is separate then the coil packs. This piece is designed to store voltage, keeping it constant in the voltage feed line to the coils. In this way the ECM can monitor the resistance of the feed line to the coil packs and throw a code if the resistance gets too high, which might indicate a coil pack going south. This little unit is an important piece and well worth changing if you are having ignition problems with your Infiniti. The capacitor ignition coil is part Infiniti # 2835189902. This piece can be had at a Nissan dealer also as they are used in multiple Nissan vehicles also.
Now Infiniti and Nissan both do a good job of hiding this coil and took a while of searching before I found it. It is included in the wiring harness itself and is factory taped around the harness, so it is hard to see. On this Q45, it is located on the passenger side of the engine, taped to the large wiring loom running done the engine and feeding the injector and coil packs. It will be at the back of the engine (on the 45) and be right on top of the loom. You should see a small wire harness coming out of the larger loom and folding back onto itself and that where the condenser is located. Cut off some of the loom tape and you will see the capacitor and it's associated plug. The old capacitor was in bad shape and was breaking apart when trying to break the connector loose from the capacitor. Be very careful because these connectors are very fragile after living in high heat conditions all of these years.
This should help you get rid of the P3012 code and any knock sensor codes that were set when the ignition problems were occurring. The capacitor was about 9.00 from the dealer, well worth the price.