Q: P0642 code, how do I know what sensors and wiring is associated with reference circuit "A"?

The only way to identify the sensors is through the factory wiring diagram and that is way too detailed to reproduce here. To resolve this issue you must have both the factory wiring diagram AND the specific "diagnostic decision tree" for Code P0642 that is also published in the manual. There are a lot of potentially relevant sensors including climate control, proximity alert, traction control, cruise control, anti-theft features, fuel injection control, anti-lock brake control, and many others. A complexity is, and the reason you need to use the decision tree from the manufacturer, is the fault is not necessarily the sensors themselves but rather a short in the wiring or a bad ground, not to mention of course the PCM itself could be faulty (although rare) or connections at the PCM culd be bad. If you do not have the manual (they are costly but worth getting), you can request a check engine light diagnostic - Code P0642 and at least, in this instance, the mechanic can walk you through the process so going forward you can be informed if a similar issue arises. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.