Having just brought a Hawkeye diagnostic scanner from Bearmach, I can honestly say this is the best way to find "which component in the system", has failed.
It saves money and time.
You plug it in in the foot well and read the screen for faults. It takes seconds. Modern day problems can have several "red herrings", in other words one problem could easily point to 14 types of fault, including dry joints or rusty connectors, especially around the xyz switch.
The Hawkeye does check automatic gear box components as well and it sounds like your engine has gone into safe "get home" mode.