What should a healthy CAN bus waveform look like on a scope?
Short answer: CAN high and CAN low should mirror each other around about 2.5 volts, with CAN high rising toward about 3.5 volts and CAN low falling toward about 1.5 volts when active, with clean, square edges.
High-speed CAN uses two wires that move in opposite directions. At rest, both sit near 2.5 volts. When a bit is sent, CAN high rises and CAN low falls by about a volt each. On a scope with both channels on the same scale and ground, you should see clean, mirror-image square waves.
Problems stand out. One line stuck at battery voltage or ground means a short. Rounded or distorted edges suggest added resistance or capacitance from damaged wiring or a failing module. Noise riding on both lines points to poor grounds or interference. A line that never moves suggests an open or a module holding the bus.
Use a fast time base to see individual bits and a slower one to see message traffic. Many scopes include a CAN preset and can decode messages. Compare with known-good captures from the same type of vehicle, because medium-speed and fault-tolerant networks look different.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.