5.8 Serial Data Bus Networks & Communication Fault Diagnosis
Key Takeaways
- ASE task E.10 requires diagnosing failures in the data communications bus network and determining needed repairs, making network diagnosis a scored A8 competency rather than an advanced-level-only topic.
- High-speed CAN uses a twisted pair in which CAN High rises from about 2.5 volts to about 3.5 volts and CAN Low falls from about 2.5 volts to about 1.5 volts, producing a differential signal that rejects electrical noise.
- Measuring about 60 ohms across CAN High and CAN Low with the key off confirms both 120-ohm terminating resistors are present, while about 120 ohms indicates one terminator or its module has dropped off the bus.
- U-codes report lost communication with a named module, so a U0100 stored in several modules identifies the PCM as the module that went silent rather than indicating multiple module failures.
- A single module shorting a bus wire to ground or to voltage takes down the entire network, which is why disconnecting modules one at a time until communication returns is the standard isolation method.
Why Network Faults Land on the A8 Test
Task E.10 sits inside Computerized Engine Controls, the largest content area on the A8 test at 26 percent. Engine control on a modern vehicle is distributed: the PCM receives vehicle speed from the ABS module, A/C request and blower load from the HVAC module, transmission range and torque requests from the TCM, and immobilizer authorization from the body or theft module — all over serial data. A network fault therefore presents as a driveability complaint: a no-start with a valid crank signal, an engine that will not leave idle, a transmission stuck in limp, or a cooling fan that never commands on.
Network Architectures
| Network | Speed | Wiring | Typical use |
|---|---|---|---|
| High-speed CAN (CAN C, ISO 11898) | 500 kbps typical | Twisted pair, terminated at both ends | Powertrain: PCM, TCM, ABS, restraints |
| Medium/low-speed CAN (CAN B) | 125 kbps typical | Twisted pair or single wire depending on design | Body, HVAC, instrument cluster |
| LIN (Local Interconnect Network) | 20 kbps max | Single wire, master/slave | Sub-systems: generator regulator, blower, wiper, some actuators |
| Legacy: SAE J1850 VPW / PWM, ISO 9141-2, ISO 14230 (KWP2000) | Low | Single or dual wire | Pre-CAN OBD-II vehicles |
| Ethernet / CAN FD | High | Twisted pair | Programming, ADAS, high-data applications on newer vehicles |
All OBD-II vehicles since the 2008 model year use CAN for the mandated emissions diagnostic communication at the DLC, on pins 6 (CAN High) and 14 (CAN Low).
CAN Signal Characteristics
High-speed CAN is a differential bus. Both wires idle at approximately 2.5 volts (the recessive state). To transmit a dominant bit:
- CAN High rises to approximately 3.5 V
- CAN Low falls to approximately 1.5 V
The receiving module reads the difference between the two wires, not either wire's absolute voltage. Because both wires are twisted together, any electrical noise couples into both equally and cancels out in the differential calculation. This is why CAN survives in an environment full of ignition and generator noise — and why an untwisted repair splice in a CAN pair can introduce intermittent communication faults.
Scope diagnosis is definitive: capture both wires simultaneously. Healthy CAN shows mirror-image square waves centered on 2.5 V.
| Scope observation | Indicated fault |
|---|---|
| CAN High and CAN Low both flat at 0 V | Bus shorted to ground |
| Both wires flat at battery voltage | Bus shorted to voltage |
| One wire switching, the other flat | One bus wire open or shorted |
| Signals present but amplitude reduced/distorted | Extra termination, water intrusion, or a partially shorted module |
| Correct waveform but no scan tool communication | Gateway module or DLC circuit fault, not a bus fault |
The 60-Ohm Termination Test
A high-speed CAN bus carries a 120-ohm terminating resistor at each physical end of the twisted pair, normally built into two modules (frequently the PCM and one other end module). With the key off and the scan tool disconnected, measuring across DLC pins 6 and 14 should read approximately 60 ohms — the two 120-ohm resistors in parallel.
| Reading | Interpretation |
|---|---|
| ~60 ohms | Both terminators present; bus wiring continuous |
| ~120 ohms | One terminating resistor missing — a terminating module is disconnected, unpowered, or the bus is open between the two terminators |
| Near 0 ohms | Bus wires shorted together or shorted to ground |
| Open/infinite | Both terminators lost, or an open in the wiring to the DLC |
This one measurement, taken in under a minute, separates a wiring fault from a module fault more efficiently than any scan-tool procedure.
Interpreting U-Codes
Network DTCs use the U (Network/Communication) prefix.
| DTC | Meaning |
|---|---|
| U0001 | High-speed CAN communication bus |
| U0073 | Control module communication bus A off |
| U0100 | Lost communication with ECM/PCM |
| U0101 | Lost communication with TCM |
| U0121 | Lost communication with ABS module |
| U0155 | Lost communication with instrument panel cluster |
| U0140 | Lost communication with body control module |
Reading the pattern is the skill. A U-code names the module that stopped talking, and it is stored by the modules that noticed. If four modules each store U0100, the PCM is the silent module. If the PCM alone stores U0121, the ABS module is the silent one. A module that is unpowered, has lost its ground, or is internally failed produces exactly this pattern — the fault is not necessarily in the bus wiring at all.
Always check power and ground at the silent module first. A blown fuse or a corroded ground at a module produces a full set of "lost communication" codes elsewhere and no bus wiring fault whatsoever.
Isolation Strategy for a Dead Bus
When no module communicates:
- Verify the scan tool and its cable on a known-good vehicle. Tool faults masquerade as vehicle faults.
- Verify DLC power and ground: pin 16 should have battery voltage, pins 4 and 5 should be chassis and signal ground. A blown DLC fuse produces "no communication with any module" with a perfectly healthy network.
- Perform the 60-ohm termination test at the DLC.
- Scope both bus wires for shorts to ground, shorts to voltage, and open-circuit signatures.
- Divide and conquer. If the bus is shorted, disconnect modules one at a time — starting with the most accessible and the most water-exposed — rechecking bus resistance or the scope pattern after each. When the bus recovers, the last module disconnected (or its harness branch) contains the fault.
- Inspect the physical layer. Water intrusion at connectors, chafed harness at body pass-throughs, rodent damage, and prior accident repairs with untwisted or improperly repaired CAN pairs are the most common physical causes.
Repair standards for CAN wiring: maintain the twist, keep repair splices short and staggered, match wire gauge, and use sealed environmental splices. An untwisted 12-inch repair section can pass a static resistance test and fail intermittently under load.
With the key off and the scan tool disconnected, a technician measures resistance across DLC pins 6 and 14 and reads approximately 120 ohms. What does this indicate?
A vehicle stores U0100 (lost communication with ECM/PCM) in the ABS, transmission, and instrument cluster modules. The PCM itself reports no communication faults and cannot be accessed with the scan tool. What should the technician check first?
On a properly operating high-speed CAN bus, what voltages should a technician expect to observe during a dominant bit?