CAN Bus and Network Fault Diagnosis

A modern vehicle is a computer network on wheels. The engine, brakes, steering, body and radio all talk over shared wires, and one failed module or rubbed wire can light up every lamp on the dash. It can look as if everything has failed at once. In most cases only one thing has.

Network diagnosis is less about the code and more about the map. You find out who is talking, who is silent and who is complaining about whom. Then you use a meter and a scope to narrow it down to the fault. You need the wiring diagram for this exact vehicle, because layouts vary a great deal.

What You Will Be Able to Do

• Explain how a CAN bus sends data, and what it should measure.
• Read lost communication codes as a pattern, not as single faults.
• Use the network diagram to decide what to test first.
• Find opens, shorts and missing terminators with a meter.
• Read CAN signals on a scope, and isolate a module that is pulling the bus down.

How CAN Works

CAN stands for Controller Area Network. Modules share two wires, CAN High and CAN Low, usually twisted together. The twist helps cancel electrical noise. Every module can hear every message, and each module uses the messages it needs.

The signal is the difference between the two wires. On a high-speed bus, at rest both wires sit near 2.5 volts. This is called recessive. When a module sends a bit, CAN High rises to about 3.5 volts, and CAN Low falls to about 1.5 volts. This is called dominant. The 2-volt difference is the data. Because noise hits both wires about equally, it does not change the difference much.

A typical high-speed bus runs at 500 kilobits a second. Most vehicles have several networks at different speeds, joined by a gateway module that passes messages between them.

Terminators

The two ends of a high-speed bus each have a 120-ohm resistor, often inside a module. They stop signals from bouncing back along the wires. Two 120-ohm resistors in parallel measure 60 ohms, so that is what you read across the bus with the power off.

Other Networks

Check the wiring diagram. Pins and layouts are maker-specific, and newer vehicles use faster versions of CAN that can look different on a scope.

Start With the Pattern

A U-code tells you that one module cannot hear another. The module storing the code is usually not the one that failed.

Run a full-vehicle scan and write down each module with its status and codes. Then draw it out.

Worked example: The ABS, transmission, steering and body modules all have U0100. The engine module does not show up on the scan. That is a pattern: four modules all lost the same one, so the bus is fine, and the engine module is offline. Do not test four modules. Check the engine module's fuses, power, grounds and connector first.

First Checks

Network faults often start with basics. Do these first.

1. Check the battery. Low voltage makes modules drop off the network. Test the battery and the charging system.
2. Ask what changed. Recent body work, a new stereo, a tracker or remote start can all be the cause. Look for an OBD dongle plugged into the connector, and remove it.
3. Check power and grounds at the silent module. Most offline modules are offline because they lost power or ground. Test under load, not just with an open-circuit reading.
4. Inspect the connectors. Look for water, corrosion and pushed-back pins. Note where the network runs through a junction or splice pack.

Testing With a Meter

The resistance check tells you a lot in two minutes. Do this with the ignition off, and the modules asleep. Some systems take several minutes to sleep. If the service information says to disconnect the battery, do that first.

Warning: Do not back-probe with sharp pins that can spread a terminal, and do not jump anything across a network. Never unplug an airbag module or its connectors without following the service procedure.

Measure across CAN High and CAN Low at the diagnostic connector, usually pins 6 and 14.

Worked example: You read 118 ohms at the connector. Two resistors would read 60, so only one is being seen. Use the diagram to find the two end modules. Unplug one, and measure across the two CAN wires on the harness side of its connector. A reading of about 120 ohms means the wiring reaches the far terminator, so this end's terminator is the suspect. Check it at the module if the service information allows that. An open means the wire is broken between the two ends, or the far terminator is gone, so repeat the check from the other end.

A 60-ohm reading does not prove the network works. It only shows the two resistors are connected. A fault that appears under load, a short to voltage or to ground, or a bad transceiver will not show on this test.

Now check each wire for a short to power and to ground. With modules unplugged, each wire should read open to ground. With the ignition on, a meter on a healthy bus reads a little above 2.5 volts on CAN High, and a little below 2.5 volts on CAN Low, because the meter averages the signal.

Testing With a Scope

A scope shows what a meter cannot. Connect channel A to CAN High and channel B to CAN Low at the connector, with the ignition on.

A healthy bus looks like a mirror image. CAN High pulses up, and CAN Low pulses down, at the same time, from a shared line near 2.5 volts.

Note that a high-speed bus usually cannot run on one wire, but some older low-speed fault-tolerant systems can. Those may keep working with one wire down, so a fault may show as a code more than a failure.

Finding the Module or Wire

If one module or a short is pulling the whole bus down, split the network.

1. Leave your scope or meter on the bus at the connector.
2. Using the diagram, turn the ignition off and unplug one module, starting with the ones that are easiest to reach, or the ones in a damp area.
3. Turn the ignition on, and watch for the signal to come back or the short to clear. Repeat for each module.
4. If the bus recovers when a module is unplugged, inspect that module's connector and wiring. Then decide if the module has failed.
5. If unplugging modules does not help, the fault is in the harness. Split it at a junction connector or a splice pack, and test each branch.

Worked example: A truck has no communication on the high-speed bus. Resistance reads 60 ohms, but the scope shows CAN Low stuck near 0 volts. It is a short to ground. You unplug modules one at a time, and nothing changes. You then disconnect the junction block behind the kick panel, and test each branch to ground. The branch to the rear reads near zero. Following it, you find the harness crushed under a seat track.

Wiring Repairs

Twist matters. When you repair a CAN wire, keep the two wires twisted as close to the original as you can, and keep them the same length. Use the method in the service information. Do not route a repair near ignition or high-current wiring. Sloppy splices can cause noise faults that are worse than the first one.

When It Happens With the Key Off

A module that stays awake can keep the network from sleeping and drain the battery. If a parasitic draw test shows current that does not fall, scope the CAN lines with the car locked and asleep. Signals that should stop will keep running. Then pull fuses one at a time, or unplug modules, and watch for the traffic to stop.

Before You Condemn a Module

A new module is a costly and slow repair. It often needs programming, security access and a paid subscription. Before you order one:

• Confirm power and grounds under load.
• Confirm that the bus is healthy at that module's own connector.
• Check for bulletins and software updates.
• Check for water damage or corrosion, and find its source.

Common Mistakes

Check Your Understanding

1. One module reports lost contact with every other module, and the rest of the network looks fine. Where do you look? That module, or its branch of the wiring.
2. You read about 40 ohms across the bus on a car with a new tracker fitted. What do you suspect? An extra terminator on the bus. Ask what was added, and remove it to test.
3. On your scope, CAN High sits near battery voltage. What is the fault? That wire is shorted to power.
4. The radio, seat and door modules are silent, but the engine, ABS and transmission modules all respond. What does that suggest? A problem on one branch or one network, or at the gateway that joins the networks.
5. You repair a chafed CAN wire. How do you run the repair? Keep the two wires twisted close to the original and the same length, follow the service information, and stay away from ignition and high-current wiring.

Key Takeaways

• Read the pattern of who lost whom. Several modules blaming one means that one is offline.
• Check the battery, add-on devices, and module power and grounds first.
• 60 ohms across the bus shows two terminators, and 120 ohms shows only one.
• A healthy CAN signal looks like a mirror image around 2.5 volts.
• Split the network to isolate a short or a bad module.
• Keep wires twisted in repairs, and prove the bus and power are good before you order a module.