Common CAN Bus Faults and Troubleshooting Methods – Part 2

2025-06-08

5. Short Circuit of CAN L to Power (Positive)

When a short circuit occurs between CAN L and the power (positive), due to the fault tolerance characteristics of the CAN bus, the entire CAN network may be unable to communicate or generate related fault codes.

Because of the short circuit of CAN L to the power, the voltage of CAN H is also set to 12V. The bus waveform for the short circuit of CAN L to the power is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

By measuring the voltage of the CAN wires, if both CAN L and CAN H wires have a voltage of about 12V, it indicates this type of fault. The voltage of CAN L in the short circuit is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

The voltage of CAN H in the short circuit of CAN L to power is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

Cause of the fault: If it is not caused by the CAN L wire shorting to an external power source, then this fault may be due to a damaged CAN transceiver inside the control module. The troubleshooting method is the same as above.

6. Open Circuit of CAN H

When the CAN H wire of a control module is open-circuited, it will prevent that control module from communicating, but other control modules can still communicate.Other control modules may read the fault code of this faulty module.If multiple control modules have an open circuit in their CAN H wires, then the communication functionality of these control modules will be affected.The bus waveform for the open circuit of CAN H is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

If the faulty control module has a terminating resistor, you can use the resistance measurement method to diagnose. Measure the resistance between CAN H and CAN L at the diagnostic interface; if it becomes 120Ω, it indicates that there is a terminating resistor open circuit. If the faulty control module does not have a terminating resistor, then you need to measure the continuity of the CAN wires of that control module.

Replacing the control module with fault code content can quickly determine whether the fault is caused by that control module itself. Additionally, you need to refer to the network diagram to find the breakpoints, as the corresponding bus hubs will be set in the entire network. Different breakpoints will affect different components, and will also determine which control modules can be diagnosed by the diagnostic tool. The distribution of the CAN network and hubs is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

7. Open Circuit of CAN L

When the CAN L wire of a control module is open-circuited, it will prevent that control module from communicating, but other control modules can still communicate. Other control modules may read the fault code of this faulty module. If multiple control modules have an open circuit in their CAN L wires, then the communication functionality of these control modules will be affected.

If the faulty control module has a terminating resistor, you can use the resistance measurement method to diagnose. Measure the resistance between CAN H and CAN L at the diagnostic interface; if it becomes 120Ω, it indicates that there is a terminating resistor open circuit. If the faulty control module does not have a terminating resistor, then you need to measure the continuity of the CAN wires of that control module. The bus waveform for the open circuit of CAN L is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

Replacing the control module with fault code content can quickly determine whether the fault is caused by that control module itself. Additionally, you need to refer to the network diagram to find the breakpoints to accurately identify the cause and eliminate the fault.

8. CAN L and CAN H Wires Reversed

When a fault occurs where CAN L and CAN H wires are reversed, generally, the control module with the incorrect connection will be unable to communicate, while other control modules will communicate normally. The schematic diagram of the reversed CAN L and CAN H wires is shown in the figure below.

Common CAN Bus Faults and Troubleshooting Methods - Part 2

Measure the voltage at the pin of the CAN wire of the suspected problematic control module to verify if the voltage is normal. Check the wiring connections against the CAN network diagram to determine if this fault exists. If it does, repair the CAN network. Replace the control module with fault code content to determine whether the fault is caused by that control module.

Source | Wiring Engineer

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Common CAN Bus Faults and Troubleshooting Methods - Part 2

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