CAN Bus &
Vehicle Network Diagnostics
Complete diagnostic procedure for Controller Area Network communication systems. Covers termination testing, oscilloscope waveform analysis, bias voltage measurement, and systematic fault isolation using divide-and-conquer methodology.
Critical Parameters
Heavy-duty commercial vehicles typically use multiple CAN buses: a powertrain bus (engine, transmission, retarder), a chassis bus (ABS, EBS, suspension), and a body bus (doors, lights, HVAC). All buses connect to the diagnostic connector (OBD/DLC) through a gateway ECU.
Typical Heavy-Duty Vehicle CAN Network Topology
The CAN bus uses differential signaling. In the recessive state, both CAN_H and CAN_L sit at 2.5 V (differential = 0 V). In the dominant state, CAN_H rises to 3.5 V and CAN_L drops to 1.5 V (differential = 2.0 V). A dominant bit always overwrites a recessive bit — this is how bus arbitration works.
CAN Bus Waveform — Oscilloscope View
Clean square edges, symmetric CAN_H/CAN_L, no ringing or overshoot. Differential amplitude exactly 2.0 V during dominant bits.
Rounded edges, asymmetric amplitudes, ringing after transitions, or one line stuck at constant voltage. Indicates wiring fault, missing termination, or EMI.
Termination Resistance — Ignition OFF
The two 120 Ω termination resistors at each end of the bus appear in parallel when measured from any point on the bus. This is the first and most important CAN bus test.
DC Bias Voltages — Ignition ON
Oscilloscope Waveform Analysis
Use math function (Ch1 - Ch2) to display the differential signal. This eliminates common-mode noise and shows the actual data signal.
Error Frame Counter Analysis
Error counters increment on each detected error and decrement on successful transmissions. A node goes bus-off at TEC > 255 to protect the network from a continuously faulty transmitter.
Divide-and-Conquer Isolation
Start from the middle of the bus and work outward. If the bus recovers when an ECU is disconnected, that ECU or its stub wiring is the fault source. Always check the connector pins before condemning the ECU.
Interactive CAN Bus Troubleshooting
Are there communication-related DTCs (U-codes) or multiple ECU fault codes appearing simultaneously?