// Wiring & Networks

CAN Bus No Communication: A Practical Diagnostic Flow

Published by DiagEdge · 9 October 2026 · 7 min read

"No communication with the scan tool" or a cluster of U-codes can feel daunting, but CAN faults respond well to a disciplined routine. The bus is just two twisted wires — CAN High and CAN Low — shared by every module, with a terminating resistor at each end. Most faults come down to power, ground, the wires, or one module dragging the bus down.

Understand the physical layer first

• CAN High and CAN Low sit at about 2.5 V at rest. During communication, High rises toward ~3.5 V and Low drops toward ~1.5 V — they move in opposite directions by roughly the same amount. That symmetry is what rejects noise.
• The bus has two 120-ohm terminating resistors, one at each end, in parallel. Measured across the bus with everything off, that's about 60 ohms. This single number is the most useful test you have.

Start here

1. Confirm the symptom. Is it the whole bus (no modules talk) or one module (everyone else talks, one is missing)? That splits the job immediately.
2. Check power and grounds at the affected module(s). A module with no power or a bad ground is "no comms" that has nothing to do with the bus wiring. Back-probe the connector and verify battery voltage and a clean ground under load.

The 60-ohm resistance test

With the ignition off (and ideally after the modules sleep), measure resistance between CAN High and CAN Low at the diagnostic connector:

• ~60 ohms → both terminating resistors present, bus wiring intact. The fault is likely a module or power/ground, not the wires.
• ~120 ohms → you're only seeing one terminating resistor. The bus is open somewhere, or one resistor/module is disconnected.
• Very low (near 0) or well under 60 → a short between the wires, or CAN High/Low shorted to each other or to power/ground.
• Infinite / open → both resistors isolated — a major break or a dead section.

Scope the bus

If resistance looks right but comms are still dead, put a scope on CAN High and Low. You want to see the two mirror-image square-ish waveforms. A flat line, both lines stuck high or low, or one line pulled to ground tells you whether the bus is being held down — often by a single faulty module.

Isolating a shorted or babbling module

If the bus is shorted or one module is corrupting it:

1. Note which modules are missing or which codes are set — the network topology narrows the suspects.
2. Disconnect modules one at a time (or by section, if the harness allows), re-measuring resistance or re-checking comms after each.
3. When disconnecting a particular module restores ~60 ohms or restores communication, that module (or its branch wiring) is your fault.

The takeaway

Work the layers in order: symptom scope → power and grounds → the 60-ohm test → scope the waveform → isolate by disconnection. The wires themselves are rarely the fault; it's usually a module, a connector, or a ground — and the resistance test points you straight at which.