CAN Bus Fault Codes and Truck Diagnostics

CAN Bus and J1939: The Physical Network

The CAN (Controller Area Network) bus is the physical communication medium that J1939 uses for module-to-module communication in heavy trucks. It consists of two wires — CAN-H (CAN High) and CAN-L (CAN Low) — that form a differential pair. Data is transmitted by creating a voltage differential between the two wires, which makes the network inherently resistant to common-mode electrical noise. The bus uses 120-ohm termination resistors at each physical end to prevent signal reflections.

In practice, the terms 'CAN bus' and 'J1939 network' are often used interchangeably in the context of heavy-duty trucks, though they technically refer to different layers of the system — CAN is the physical and data-link layer; J1939 defines the higher-level network management, message format, and parameter standards. A fault in the CAN bus (broken wire, short circuit, or termination resistance problem) produces J1939 communication fault codes in the modules that depend on it.

CAN Bus Fault Diagnosis: The 60-Ohm Test

The most common field test for CAN bus health is measuring the bus resistance at the 9-pin diagnostic connector with the ignition off and no modules powered. With both 120-ohm termination resistors in the circuit and no shorts, the measured resistance between CAN-H (pin C) and CAN-L (pin D) on the J1939 diagnostic connector should be approximately 60 ohms (two 120-ohm resistors in parallel). Deviation from this value indicates a specific fault type.

A measurement significantly above 60 ohms (120 ohms, infinite, or open) indicates that one termination resistor is missing or the bus has a break between the test point and a terminator. A measurement significantly below 60 ohms (near zero) indicates a short circuit between CAN-H and CAN-L — this prevents any module on the bus from communicating and typically causes all modules to log communication faults simultaneously. A measurement of approximately 60 ohms confirms the physical network is intact, directing the diagnostic focus toward software or module power issues rather than bus hardware.

Wiring Damage, Chafing, and Connector Corrosion

CAN bus wiring in heavy trucks runs through the engine compartment, across the frame, and into cab and trailer connections — all environments that expose it to heat, vibration, road spray, and physical abrasion. Common CAN bus fault causes include: chafed wiring where the harness contacts a sharp edge or hot surface (creating an intermittent short or open), corroded connector terminals (particularly at the 9-pin diagnostic connector, the ECM connector, or any connection exposed to moisture), and wiring damaged by rodent activity.

Intermittent CAN bus faults — ones that appear only under vibration at highway speed or during specific operating conditions — are particularly difficult to diagnose with static tests. A flexible test drive while monitoring J1939 communication status in a diagnostic tool can sometimes catch intermittent faults that a bench inspection misses. Harness sections that run near exhaust components or near wheel-end equipment deserve extra scrutiny for heat-related or vibration-related damage.

Multi-Network Trucks and Network Isolation

Modern heavy trucks often have multiple CAN networks running at different data rates or for different purposes — a high-speed J1939 network for powertrain and safety modules, a lower-speed network for body and cab functions, and potentially separate networks for trailer connections (the electronic trailer ABS connection uses J1939 over a separate physical circuit). Faults on one network do not necessarily affect other networks, which is why a communication fault on the trailer ABS J1939 connection appears isolated from the tractor's main network faults.

When diagnosing CAN bus or J1939 communication faults, knowing which physical network segment the affected modules share helps isolate the fault location. The OEM diagnostic tool or service documentation will identify which modules share which network segment and where the segment's termination resistors are located.

Related Fault Code Pages

Sources

SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source

FAQ

What is the most common physical cause of a CAN bus fault on a heavy-duty truck?

Damaged wiring is the most common cause — chafed insulation where harnesses flex near the firewall or frame, corroded connectors in harsh environments, and loose or pulled pins in multi-pin connectors are typical culprits. Termination resistor failure is the second most common cause. Both are physical, low-cost items to check before suspecting module failures.

Can an aftermarket device connected to the diagnostic port cause CAN bus faults?

Yes. Aftermarket telematics, ELD adapters, and diagnostic dongles that are not properly matched to the bus load or that have failed internal CAN transceivers have been documented as causes of bus communication problems. If CAN communication codes appeared after new hardware was connected to the diagnostic port, disconnect the device and recheck before pursuing other causes.

Are there multiple CAN networks on a heavy truck, or is there just one J1939 bus?

Modern heavy trucks often have multiple CAN networks operating at different speeds for different functions. The primary powertrain J1939 network (typically 250 Kbps) carries engine, transmission, ABS, and instrument cluster communication. Some trucks have secondary high-speed CAN networks for specific systems or chassis functions. A fault on one network may not affect modules on the other, which is why confirming which network segment is affected matters for diagnosis.