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How to diagnose intermittent CAN-bus error frames causing ECU communication dropouts?

Alex Rivera

Asked 2 hours ago

• 245 views

I'm diagnosing a 2021 fleet vehicle that randomly sets U0100 (Lost Communication with ECM/PCM) and drops into limp mode.

When measuring with a digital multimeter, I get roughly 2.5V on both CAN-H and CAN-L at idle, and exactly 60 Ohms across Pin 6 and 14 with the battery disconnected.

However, when viewing the signal on an oscilloscope during road-test vibration, I occasionally see the differential voltage spike to approximately 4.5V with messy ringing edges.

How can I determine whether this is caused by an inductive noise spike from an ignition coil, a bad terminating resistor, wiring damage, or an internal transceiver fault inside a module?

Current Measurements

CAN-H

~2.5V

CAN-L

~2.5V

Termination

60 Ω

#CAN-bus #Oscilloscope #Diagnostics #ECU

1 Answer 245 Views

3 Answers

Answers from the community

Best Answer

Accepted by Alex Rivera

A 60 Ohm resistance across CAN-H and CAN-L with the battery disconnected generally indicates that both 120 Ohm terminating resistors are present.

The next step should be to capture the CAN differential signal while the fault is actually occurring. Compare CAN-H and CAN-L individually as well as the differential waveform.

If the ringing occurs only during vibration, inspect the physical harness first. Pay particular attention to connectors, shield grounds, splices and areas where the harness contacts brackets or the engine.

CAN-H  ─────╱╲─────
CAN-L  ─────╲╱─────

Check:
1. Differential voltage
2. Common-mode voltage
3. Edge ringing
4. Error frames
5. Harness movement

You can also perform a wiggle test while monitoring the oscilloscope. If the waveform changes when a particular section of harness is moved, isolate that section before replacing the ECU.

Michael Chen

Automotive Diagnostics Expert • Answered 1 hour ago

I would also check the grounding and power supply of the modules involved in the communication fault.

A module can appear to have a CAN communication problem when the actual problem is a voltage drop on its power or ground connection.

Capture the module supply voltage simultaneously with the CAN waveform during the event.

Kenji Sato

Answered yesterday

Before replacing the terminating resistor, remember that the 60 Ohm reading suggests that the termination resistance is probably not the primary issue.

I would inspect the connector pins, harness routing and shielding first, particularly if the fault is related to vehicle vibration.

Daniel Brooks

Answered yesterday

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How to diagnose intermittent CAN-bus error frames causing ECU communication dropouts?

I'm diagnosing a 2021 fleet vehicle that randomly sets U0100 (Lost Communication with ECM/PCM) and drops into limp mode. When measuring with a digital multimeter, I get roughly 2.5V on both CAN-H and CAN-L at idle, and exactly 60 Ohms across Pin 6 and 14 with the battery disconnected. However, when viewing on an oscilloscope during road test vibration, I occasionally see the differential voltage spike to 4.5V with messy ringing edges. How can I determine whether this is an inductive noise spike from an ignition coil, a bad terminating resistor, or an internal transceiver fault inside a module?

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