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A lorry can arrive with a warning lamp, reduced power and a long list of fault codes, yet the root cause may be one damaged pair of CAN wires or a control unit pulling down the network. This CAN bus guide is written for workshops that need to establish whether the fault is in the network, a module, the power supply or the diagnostic process before replacing costly parts.
What CAN bus does in a heavy vehicle
Controller Area Network, usually called CAN bus, allows electronic control units to exchange information over a shared communication circuit. Rather than every unit having a separate wire to every other unit, multiple controllers use the same network. On a modern EURO 5 or EURO 6 lorry, that may include the engine ECU, transmission controller, instrument cluster, ABS/EBS, body control module, tachograph, AdBlue/SCR system and diagnostic socket.
The practical benefit is reduced wiring and faster data exchange. The practical workshop consequence is that a fault in one section can affect functions that appear unrelated. A communication issue reported by the engine ECU does not automatically mean the engine ECU has failed. It may be receiving incomplete information from another controller, or it may be unable to see the network at all.
Most heavy vehicles use more than one network. A high-speed CAN network carries time-critical data for powertrain and braking functions. A lower-speed network may handle cab, lighting and convenience functions. Gateways connect these networks and manage which messages pass between them. This is why a scan tool may communicate with some modules but not others.
CAN bus guide: understand the circuit first
A typical high-speed CAN circuit uses two twisted wires: CAN High and CAN Low. Twisting helps reject electrical interference, which matters on vehicles with long harnesses, high-current equipment and demanding operating environments. The two lines carry opposite voltage changes, allowing control units to read the difference between them rather than relying on one wire alone.
At rest, both lines are commonly close to 2.5 V on a healthy high-speed network. During communication, CAN High rises and CAN Low falls. Exact readings vary with vehicle design, network load and meter response, so a multimeter is useful for an initial check but not for proving signal quality. An oscilloscope provides the clearest view of waveform shape, noise, reflections and missing termination.
A correctly designed high-speed network normally has two 120-ohm termination resistors, one at each end of the main bus. With the vehicle powered down and relevant control units allowed to sleep, a resistance check across CAN High and CAN Low at a suitable point commonly reads about 60 ohms. A reading near 120 ohms can indicate one missing termination path. A very low reading can point to a short circuit, a failed module or incorrect measurement conditions.
Do not treat the 60-ohm figure as a universal pass or fail test. Some vehicle architectures use gateway-controlled networks, separate branches or non-standard access points. Always confirm the wiring layout and test point for the vehicle in front of you.
Symptoms that point to a network fault
Network faults rarely present in exactly the same way twice. A lorry with an intermittent CAN issue may start normally in the workshop and fail only when vibration, moisture or temperature changes affect the harness. The fault may also occur after body repairs, accessory installation, battery replacement or water ingress around a connector.
Common signs include multiple communication DTCs, warning messages from several systems, an unavailable diagnostic connection, gauges dropping out, transmission or engine derate, and modules that appear and disappear during a scan. When several controllers simultaneously report that they cannot communicate with one specific unit, focus first on that unit’s supply, earths, connector condition and network branch.
A single module communication code is different from widespread network loss. If one ECU is absent but the rest of the network remains stable, the fault is often local to that ECU, its connector or its feed. If nearly every controller logs communication errors, inspect the main network, gateway and battery voltage before condemning individual units.
A practical diagnostic sequence
Start with a full vehicle scan and save the report before clearing anything. Record which modules respond, which do not, and whether faults are current, stored or intermittent. Vehicle-specific diagnostic software is valuable here because generic scanners may show only broad communication codes without identifying the affected branch or gateway.
Check battery condition and charging voltage next. Low system voltage is one of the fastest ways to create misleading communication faults. Inspect main battery connections, chassis earths, ECU feeds and fuses under load where possible. A clean voltage reading with no load does not prove that a corroded connection can support a working network.
Then inspect the accessible harness. Look for stretched twisted pairs, crushed sections, poor repairs, corrosion, water entry and non-standard joins. Pay close attention to areas around batteries, chassis rails, cab tilt points, engine harness routes and connectors exposed to road spray. A CAN pair repaired with unsuitable wire, excessive untwisting or poor joint protection can create an intermittent fault that is difficult to reproduce.
With the network asleep and the manufacturer’s procedure followed, measure resistance between CAN High and CAN Low. If the result is abnormal, isolate branches methodically rather than unplugging random controllers. Disconnecting modules one at a time can identify a unit or branch that is loading the bus, but doing so without a wiring diagram may create additional codes and waste time.
If resistance is plausible but the complaint remains, use a scope. A healthy signal should show consistent, opposing activity on CAN High and CAN Low. Rounded edges, excessive noise, reflections, unequal amplitudes or one line held high or low all provide useful direction. Compare readings at the diagnostic connector, gateway and suspected ECU where access and vehicle information allow.
Why fault codes alone are not enough
CAN-related DTCs identify a communication problem, not necessarily its cause. “No communication with SCR control unit” may be caused by a failed controller, but it can also result from a blown fuse, poor ground, damaged CAN wiring, water in a connector, a gateway fault or low voltage during cranking.
This is where workshop discipline saves money. Replacing a controller without checking supply, earth and network integrity can turn a straightforward wiring repair into an expensive return visit. It also risks programming delays if the replacement unit requires coding, parameterisation or security access.
Use live data to support the diagnosis. If a module communicates intermittently, watch supply voltage, ignition status, network state and related sensor values while moving the harness carefully. If the issue is temperature-related, allow the vehicle to reach the conditions under which the customer reports the fault. A fault that only occurs after a long run cannot always be confirmed on a cold vehicle at the bench.
CAN bus repairs that last
A correct repair protects both electrical continuity and network characteristics. Maintain the twist of the pair as closely as possible, use appropriate automotive-grade cable and connectors, and seal repairs against moisture and vibration. Avoid long untwisted sections and improvised joins. These may work at first, then create signal reflections or corrosion failures later.
After repair, clear faults only once the underlying cause has been addressed. Re-scan every affected controller, confirm stable communication, check that warning lamps remain out and road-test the vehicle where safe and appropriate. For systems related to braking, steering, engine management or emissions, follow the manufacturer’s repair and verification procedure.
Network intervention tools should never be used to defeat legally required emissions or safety functions. A workshop should diagnose the actual electrical or system fault, repair it properly and ensure the vehicle remains compliant for road use.
Choosing the right diagnostic equipment
The best tool depends on the job. A quality multimeter is essential for power, earth and basic resistance checks. A two-channel oscilloscope is far more effective for confirming CAN signal integrity. Vehicle-specific diagnostic equipment adds module identification, guided tests, live data, coding functions and manufacturer-level fault detail.
For mixed fleets, coverage matters as much as features. DAF, MAN, Iveco, Mercedes-Benz, Scania, Renault and Volvo vehicles use different architectures, connectors and diagnostic routines. Before purchasing equipment, check supported model years, protocol coverage, update arrangements and whether the tool can access the systems your workshop actually services. Truckdiag supplies specialist diagnostic equipment for commercial vehicle workshops that need compatibility-led solutions rather than generic code reading.
A CAN fault is often found faster when the technician stops treating every stored code as a separate failure. Start with the network layout, prove power and earth, test the bus correctly and isolate the affected branch with evidence. That approach protects repair time, avoids unnecessary ECU replacement and gets the lorry back into service with confidence.
