This is true if you are working on a MB or a KIA. These days with all the on board computers and systems, the first thing the tech grabs for is a scan tool.
The scan tool is only as good as the information is available for it from the vehicle. As one of my instructors would say “garbage in, garbage out”
So it should become old habit to first check battery voltage, and voltage drops. Systems will not work properly or give false fault codes if there is low voltage available to the system.
For instance a 2008 BMW 535i was constantly throwing up these codes:
DME (Engine-Motor Electronics)
2EE6 Digital Motor Electronics (DME): coolant temperature sensor, measurement range present
ABS-DSC (ABS Dynamic Stability Control Premium)
5EBA Dynamic Stability Control (DSC): LWS (Steering Angle Sensor) plausibility Absent
After the tech worked on this BMW for week doing different tests and checks, turns out it had a bad battery!
So when you start the diagnosis start with checking the battery. When you get a fault code, don’t condemn the component until checking the battery power and chassis grounds for the system and component.