Diagnosing CAN Bus Communication Faults

When you see U codes — communication fault codes — on multiple modules, do not start chasing individual system faults. U codes mean modules cannot talk to each other on the network. One network problem generates dozens of fault codes across every system that depends on shared data. Fix the network first. Every other code may clear on its own once communication is restored.

Disconnect the battery. Measure resistance between OBD-II DLC pin 6 (CAN High) and pin 14 (CAN Low). You should read approximately 60 ohms — this is the two 120-ohm terminating resistors in parallel. 60 ohms means the bus backbone is intact and both terminators are present. 120 ohms means one terminator is missing or disconnected — identify which module contains it using the manufacturer schematic. Below 60 ohms means a short is loading the bus. Above 120 ohms or OL means the bus wiring itself is open somewhere.

Modern vehicles have multiple CAN networks — High Speed for powertrain, Medium Speed for body, Low Speed for non-critical systems, plus LIN sub-buses. The scan tool tells you which modules set U codes. Map those modules to their network using the manufacturer diagram. If only powertrain modules have U codes, the HS-CAN bus is the problem. If only body modules are affected, check the MS-CAN bus. Do not test the wrong network.

If the resistance reading is below 60 ohms, a module or wiring fault is shorting the bus. Disconnect modules from the network one at a time while measuring resistance at the DLC. When you disconnect a specific module and the resistance jumps back to 60 ohms — that module has an internal short that is pulling down the entire network. A single shorted module can shut down communication for every other module on that bus. Replace the shorted module and verify communication is restored.

An open in the CAN bus wiring splits the network into two segments. Modules on one side of the break can talk to each other but not to modules on the other side. The pattern of which modules can communicate and which cannot tells you approximately where the break is. The break is between the last communicating module on one side and the first non-communicating module on the other. Check the wiring and connectors in that section for damage, corrosion, or an unplugged connector.

Most modern vehicles have a gateway module that bridges the separate CAN networks together. If the gateway fails, modules on different networks cannot share data even though each individual network is functioning perfectly. You see U codes between powertrain and body modules but each group communicates within itself. Check gateway module power, ground, and connectivity before tearing into bus wiring. A bad gateway power supply or ground can mimic a network-wide fault.