You would need to purchase an OBD scanning /code reader tool. OBD= On Board Diagnostics. All modern vehicles use this standardized diagnostic procedure. They are relatively cheap $25-30 at any auto parts store. It is your/the technicians interface to the cars computer. Working on any vehicle newer than 1995/96 is almost impossible to do without one. O2 sensors are one of the easier repairs, the OBD tool will Dx for you, the actual repair can vary anywhere from 20 minutes diy to “special tool” purchase, accessibility issues and the OEM(original equipment manufacturer) compatible replacement part cost. It may be possible that the OBD tool you purchase might be able to help resolve the gauge issues you mentioned. Hope this steered you in a successful direction. Good Luck

sounds like there is a “switched hot” = there is a constant hot wire connected to a switch and when switch is opened/off the power is disconnected other devices and circuits, likewise when the switch is closed/on the power to the fixture and the other devices turn on. As for the the the code regulations mentioned YES, THEY SHOULD be made compliant as per qualified electrician. He will be able to fix the switched hot like he was eating cake..