Vehicle diagnostic connectors and cables on a workbench with participants seated behind them during WTA CAN training.

Building Vehicle Network Diagnostic Skills for Fleet Maintenance

Nov 3

4 min read

When a driver reports that gauges dropped out and warning lights appeared, the vehicle may look normal by the time it reaches the shop. Dispatch needs to know whether another vehicle is required. The technician has communication fault codes, an intermittent complaint and a decision to make: what evidence would justify a repair, a module replacement or escalation?

For fleet maintenance managers, that is the useful test of vehicle network training. Can the team distinguish a loss of communication from a failed component, explain what remains uncertain and leave a work order another technician can continue? A course outline alone cannot answer those questions. The shop needs observable diagnostic skills and a consistent standard for recording the evidence.

Understand the communication path

Multiplexing lets vehicle controllers share information through communication networks. On a Controller Area Network, or CAN, nodes broadcast messages on a shared bus. Message processing and the physical connection are different parts of that system; high-speed CAN commonly uses a differential pair of wires. Texas Instruments explains these fundamentals. Knowing the arrangement helps a technician reason about what a missing message does, and does not, establish.

The vehicle may contain several networks rather than one bus connecting everything. J1939 is a CAN-based higher-layer protocol family, widely associated with commercial vehicles, not another name for every vehicle network. Technicians need the actual vehicle architecture and current service information, not a memorized diagram assumed to fit the fleet.

Off-vehicle reporting adds another distinction. A telematics dashboard can lose data without proving that an electronic control unit, or ECU, has failed. For example, Geotab describes its Vehicle Disconnected status as a device-to-server connection issue that can involve wireless connectivity or device power. The manager should ask which communication path is affected before assigning the problem to the vehicle network.

Refuse truck with a colorful network-style overlay across its cab and body. — Conceptual illustration of connected vehicle systems, not a wiring diagram.

Make the work order support the diagnosis

A communication code is not a parts order. An International J1939 troubleshooting bulletin links complaints such as missing gauges with communication faults and checks of power, grounds, wiring and connectors. It notes that intermittent faults may disappear during testing. This model-specific historical example illustrates reasoning, not a universal procedure. Actual work requires current VIN- and model-specific OEM instructions, approved tools and safe-work practices.

Set an expectation that the work order preserves the original complaint, the conditions when it occurred and the available scan or code evidence before that evidence is cleared or changed. Record the vehicle identification, relevant equipment configuration and the service information used. A bare note saying “lost communication” leaves the next person to rediscover the context.

The diagnostic record should then connect the proposed action to the findings. What was checked, under which conditions, and what did the result rule in or leave unresolved? If replacement is recommended, what evidence supports the component diagnosis rather than a supply or network problem? If the issue must go to an OEM or specialist, send the findings and remaining question with it. That makes escalation a usable handoff rather than a restart.

An unresolved complaint also needs an owner. Record who will review the evidence, what additional information is needed and whether the next action belongs with the shop, OEM support or telematics provider. Vehicle release should follow the fleet’s applicable requirements, not an assumption that a quiet dashboard means the original fault is gone.

Check the skills behind the diagnosis

Use a few recent communication-related work orders to identify gaps. Ask technicians to demonstrate the following on equipment representative of the fleet, within their authorized scope:

Can they explain the relevant network layout? Ask them to identify the affected controllers and communication path on the correct diagram, and explain which other symptoms would help narrow the problem.
Can they preserve useful evidence? Look for the original complaint, operating conditions, scan results and a clear distinction between a fault observed now and one recorded previously.
Can they verify the electrical foundations? Ask how current OEM information directs checks of powers, grounds, wiring and connectors, and where their training or authorization requires them to stop.
Can they justify the tool and test? A technician should explain what the selected scan function, diagnostic software or network analyzer can establish, and what its result cannot prove.
Can they close the diagnostic record? Require the supported cause, repair performed and verification result. If the complaint was not reproduced or the cause remains uncertain, the record should say so.

Assess the reasoning as well as the repair

A useful practical assessment is an instructor-controlled communication-fault exercise on an appropriate training system or vehicle. Ask the technician to interpret the complaint, select the relevant service information, propose justified tests and produce a short work-order record. The instructor can observe whether the technician preserves evidence, follows the authorized procedure and changes the hypothesis when the findings do not support it.

Ask the technician to explain what would change the next decision if the scan results and reported symptom do not agree. Score the reasoning against predefined criteria, not confidence or speed alone.

Evaluate the explanation and documentation alongside the outcome. Clearing a code is not the same as demonstrating that the complaint has been resolved. The final record should identify the verification performed under the applicable procedure and its result, including any conditions the exercise could not reproduce. Managers can then discuss a specific skill gap rather than concluding that someone simply needs “more electrical training.”

Build training around the fleet

WTA’s Multiplex and PLC Controllers Technician Training combines classroom instruction with hands-on work over 16 hours. The course covers CAN, LIN and Ethernet fundamentals, diagrams and network topologies, multiplex fault identification, diagnostic software and network analyzers. Introductory PLC logic and ladder concepts support fleet diagnostic work.

For a scheduled training option, see WTA’s CAN and Telematics Systems Training, December 10–11, 2026, at Cerritos College in Norwalk, California. View course details and contact WTA about registration.