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A lorry can arrive with an AdBlue warning, an intermittent gearbox message and a no-start condition, yet the root cause may be one damaged CAN wire or a module pulling down the network. Effective CAN fault resolution means proving the condition of the communication network before replacing expensive control units.
For workshops and fleet maintenance teams, this is not an academic exercise. A CAN fault can stop diagnostics communicating, place the vehicle in reduced-power mode or create a long list of unrelated-looking fault codes. The quickest repair comes from a structured test process, the correct wiring data and an understanding of which modules are meant to be awake at the time of testing.
Why CAN faults cause misleading symptoms
Controller Area Network, normally shortened to CAN, allows control units to exchange information across a shared two-wire circuit. On a modern EURO 5 or EURO 6 lorry, the engine ECU, SCR system, instrument cluster, transmission, ABS/EBS, body controller and diagnostic socket may all rely on one or more CAN networks.
When communication is interrupted, one module may report a fault in another system simply because expected data is missing. An engine ECU may log a missing message from the NOx control unit. The dash may show an emissions warning because it cannot receive valid SCR information. A diagnostic tool may list multiple control units as unavailable. None of this proves that every named component has failed.
That distinction matters. Replacing a dosing module, NOx sensor or ECU without confirming network integrity can add cost and leave the original fault untouched. CAN diagnosis should begin with the network, its power supplies and its physical condition, then move towards individual modules only when the evidence supports it.
Start CAN fault resolution with the fault pattern
Before reaching for a multimeter, record the vehicle’s symptoms and scan every accessible ECU. Look for a pattern rather than focusing only on the first code displayed. A single missing-message code may point to a local supply, connector or module issue. Several missing-message codes across unrelated systems often indicate a network wiring, termination or voltage problem.
Also establish whether the fault is permanent or intermittent. A fault that appears after rain, after cab tilt, during vibration or once the vehicle is warm gives useful direction. Water ingress at chassis connectors, loom chafing near brackets and stretched wiring around cab movement are common causes on commercial vehicles.
Check whether diagnostics can communicate with every module. If the scan tool connects to the engine ECU but not to ABS, SCR and the body controller, identify which network those modules share. If no module communicates through the diagnostic connector, first check diagnostic socket power, earth and the relevant CAN pins. A failed tool connection does not automatically mean the vehicle CAN network has failed.
Verify power and earth before testing the data wires
A control unit without a stable supply cannot communicate correctly. This simple point prevents a great deal of wasted time. Before condemning CAN High or CAN Low, verify battery voltage under load, ignition feed, permanent feed and earth quality at the affected ECU.
Use a voltage-drop test where possible instead of relying only on continuity. An earth wire can show continuity with a meter while becoming unstable when the module is operating. Corroded grounds, weak battery connections, damaged fuse-box terminals and water-contaminated connectors can all create communication faults that appear to be wiring problems.
On vehicles with multiple batteries, check the condition and balance of the battery set. Low system voltage during cranking can cause a number of ECUs to reset or fail to initialise. If the network recovers once the engine is running, do not ignore the starting-voltage condition.
Inspect connectors where the fault is most likely
Physical inspection still earns its place in a modern diagnostic bay. Disconnect power safely and inspect connector pins for spread terminals, green corrosion, pushed-back pins, damaged seals and signs of heat. Pay particular attention to areas exposed to road spray, battery acid, heat and repeated movement.
For SCR-related communication faults, inspect connectors and looms around the dosing control equipment, NOx sensors, tank assembly and chassis routing. For cab-related faults, examine loom sections around the cab hinge, bulkhead and fuse-board area. The exact weak points depend on the make and model, so correct vehicle-specific wiring information is essential.
Check termination resistance correctly
Most high-speed CAN networks use two 120-ohm terminating resistors, normally positioned at opposite ends of the network. With the ignition off, key removed where required and the network allowed to go to sleep, measure resistance between CAN High and CAN Low at a suitable access point.
A reading close to 60 ohms generally indicates that both terminating resistors are present. A reading near 120 ohms can indicate one missing termination, an open circuit to one end of the network or a disconnected module that contains a terminating resistor. A very low resistance may indicate a short circuit, incorrect wiring or a failed component affecting the bus.
These readings are guides, not a complete diagnosis. Some vehicle architectures use gateways, separate CAN branches and different network designs. Always refer to the correct wiring diagram and test specification for the lorry in front of you. Measuring resistance on a live network can also produce misleading results and may risk damage to equipment.
Use voltage checks to find shorts and opens
With the network active, CAN High and CAN Low should both show a changing voltage signal around a common level. A standard multimeter may display average values only, but it can still help identify obvious problems such as a line fixed at battery positive, fixed at earth or showing no activity at all.
An oscilloscope provides a clearer picture. A healthy high-speed CAN signal normally shows two mirrored waveforms. Reflections, distorted edges, excessive noise or a line that does not switch can reveal poor termination, damaged cable, corrosion or a module fault. This is especially useful for intermittent faults that are not present when the vehicle is stationary.
Do not use a test lamp on CAN wiring. Its load can damage the circuit or create further faults. Use high-impedance measuring equipment and back-probe carefully to avoid opening terminals or causing poor contact after testing.
Isolate a module only with a plan
If supply checks, wiring inspection and signal testing suggest that a module is pulling down the network, isolate modules in a logical order. Start with units linked to recent work, water exposure or the first stored communication fault. Disconnect one module at a time, then recheck resistance, network voltage and diagnostic communication.
This process must account for vehicle safety systems and manufacturer procedures. Disconnecting an EBS, airbag, steering or engine-related ECU can create additional faults and may require calibration or code clearing afterwards. Maintain battery support where required and record every change so that the final repair can be verified.
A module is not automatically defective because communication returns when it is unplugged. Its power or earth circuit may be at fault, the connector may be contaminated, or a shorted branch of wiring beyond that module may be influencing the result. Confirm the fault at the connector and harness before ordering parts.
Repair standards matter on commercial vehicles
A quick twisted-pair repair with poor joints can return the lorry to the road briefly and create a repeat fault weeks later. CAN wiring relies on controlled pair routing and twist rate to resist electrical interference. Keep repairs as short as possible, maintain the original wire size and twist, and use properly sealed joints suitable for the vehicle’s operating environment.
Avoid untwisting the pair over long distances. Keep CAN wiring away from high-current cables where the original routing permits, and secure the repaired loom against vibration and abrasion. Once the repair is complete, carry out a physical pull test, check for fault code return and road-test the vehicle if it is safe to do so.
For recurring faults, inspect the cause of the damage rather than repairing the same section repeatedly. A loom rubbing on a chassis edge, a loose mounting bracket or water entering through a damaged seal will defeat even a good electrical repair.
When diagnostic equipment makes the difference
Basic electrical checks are essential, but vehicle-specific diagnostic equipment shortens the process considerably. It can identify installed control units, display live communication status, confirm supply-voltage data and show which messages are missing. On some systems, guided tests and wiring references can narrow the fault to a branch or connector location.
The right tool depends on the fleet and work type. A workshop handling DAF, MAN, Iveco, Mercedes-Benz, Scania, Renault and Volvo vehicles benefits from coverage that can read manufacturer-level data rather than generic engine codes alone. Truckdiag supplies specialist diagnostic and programming equipment for commercial vehicle work, where compatibility and accurate system access are central to a profitable repair.
Be cautious with any apparent SCR or emissions-system fault. A CAN communication issue can trigger warnings relating to AdBlue, NOx or derate conditions even when the emissions components themselves are serviceable. The correct repair is to restore the vehicle’s original communication and emissions-control operation, then confirm that all relevant system tests complete successfully.
A disciplined repair prevents repeat downtime
CAN faults reward methodical diagnosis. Read all modules, establish the fault pattern, test supply and earth, inspect the physical network, verify termination, assess signal quality and isolate components only when the measurements justify it. Skipping steps may feel quicker, but it usually leads to unnecessary parts and another booking when the lorry returns.
A final scan after repair should show stable communication, relevant fault codes cleared or inactive, and no new faults introduced during testing. When the network is restored properly, the repair does more than remove a dashboard warning – it gives the vehicle a better chance of staying on the road where it earns its keep.
