CAN Bus Diagnostics: CAN-H, CAN-L and U-Code Faults

Search intents covered by this guide

Different phrasings of the same technical problem are consolidated into one substantial guide rather than split into thin pages.

Verify module power first

• A module shown offline does not prove the CAN bus is faulty. Verify power, ground and wake/ignition feeds under load.
• Low battery or charging voltage can create multiple communication codes at once.

Separate the physical layer

• Viewing CAN-H and CAN-L together can reveal clues for opens, shorts, reflections and interference.
• Evaluate termination only with the circuit de-energized and at a topology-appropriate point.

Topology and gateways

• Map which modules are unreachable. If an entire subnetwork is lost, consider gateway, shared power or shared wiring faults.

Verify the repair

• Clearing codes alone is not proof. Recheck stability under network load, sleep/wake cycling and the original condition.

6-step diagnostic workflow

• Verify vehicle and system identity.
• Preserve codes and first-event data before clearing.
• Compare commanded values with actual feedback under the same condition.
• Verify power, ground, signal/network and the physical system with independent measurement.
• Isolate root cause and justify the parts decision with evidence.
• After repair, reproduce the original condition when practical and retest.

Related technical guides

Service and application decision

Connect general diagnostic logic to the exact vehicle application. Use manufacturer service information and verified vehicle identity for exact pins, torque values, pressure thresholds, calibration and procedures.

Source and verification boundary

This guide explains system logic and diagnostic sequence. Application-specific exact values are not invented without sources.