A CAN bus short will generally stop everything and will often prevent the DTCs from being read, but lots of CAN errors may be a break or short.
With the ignition on, the CANbus wires will be at around 2.5 V if there is no traffic. Of course, there will be traffic, and the voltage on CAN-H will be a bit above 2.5 V, and CAN-L will be about the same amount below CAN-L. If the CANbus voltages are in line with that, it's working fine.
Of course, a module can be not working, or missing, and that won't change the voltage much at all, and will be difficult to detect unless you can read the CAN messages.
With the ignition off, there may be a pause of several minutes until the CANbus traffic stops.
With the battery disconnected, there should be close to 60 Ohms between the CAN wires, and a much larger resistance, at least 1 kOhm, between either CAN wire and ground or supply. It doesn't need to be precisely 60 Ohms. Anywhere from 50 - 80 Ohms will be fine. A real fault will take it outside that range.
The CANbus has a terminator of 120 Ohms at each end so the two terminators appear in parallel when measured with a multimeter. If you measure around 120 Ohms, there is a break in the CANbus somewhere.