NETWORK / CAN

CAN BUS FIRST-LOOK

View CAN High and CAN Low together to screen the physical layer for activity, symmetry, noise and clipping.

ESTIMATED SETUP2–5 min2 channels

WHY BEFORE DISASSEMBLY

CAPTURE THE SYSTEM
AS IT FAILED.

Randomly unplugging modules can wake the network, add faults and disturb the original condition. A non-invasive two-channel capture establishes whether the bus is active and electrically plausible before topology-based isolation begins.

Reading the signal

Good waveform

On the identified high-speed CAN segment, synchronized CAN-H and CAN-L show complementary dominant transitions and return to the expected recessive bias. Examine the differential signal as well as each line.

Faulty waveform

A stuck line, collapsed differential signal, severe ringing or unequal transitions can indicate wiring, termination or transceiver problems. A healthy-looking physical waveform does not prove that message content is correct; use topology, decoded traffic and DTC evidence together.

Common mistakes

AVOID THE
FALSE CLUE.

Testing with the connector unplugged when the fault only appears under load.
Using a generic voltage value instead of the vehicle-specific specification.
Skipping power, ground and reference checks before condemning the component.
Moving the harness or clearing codes before capturing the original evidence.
Replacing a part without repeating the test.

PREPARE THE TEST

✓ Identify the exact CAN network and test point✓ Confirm scope grounds are common-safe✓ Set both channels to the same range✓ Do not assume OBD pins represent every network

STEP-BY-STEP PROCEDURE

Connect Channel A to CAN High and Channel B to CAN Low at the approved test point.
Wake the required network and capture both channels on identical voltage and time scales.
Inspect recessive and dominant levels, edge quality, complementary movement and idle behavior.
Use channel math only after confirming both raw channels are correct.
If abnormal, compare at another documented point and use the topology to isolate branches in a controlled order.
Use resistance testing only with the network powered down and after following the vehicle procedure.

WHAT THIS TEST CANNOT PROVE

A healthy-looking physical waveform does not prove message content or module logic. Voltage and termination differ among network types.

⚠ SAFETY / CONNECTION CAUTION

Never use an earth-grounded probe arrangement that can short network conductors. Verify the scope’s ground architecture first.

STOP BEFORE CONDEMNING THE PART

Repeat the capture, verify the circuit under load and use vehicle-specific specifications. Follow with the most direct confirmation test indicated by the evidence.

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