What are the common causes of U0021?

An open in the low speed CAN positive wire. A broken or corroded low speed CAN positive connector pin. A module that has lost its low speed CAN positive connection. Harness damage that cut the low speed CAN positive conductor.

Read the code and check module communication status on a scan tool. Test the low speed CAN positive wire for continuity end to end. Check the low speed CAN positive connector pins for corrosion or backing out. Measure bus resistance across the low speed CAN terminals. Repair the open in the low speed CAN positive circuit. Clear the code and retest.

U0021

Low Speed CAN Communication Bus (+) Open

System: Network communication (U code)
Lamp: Check Engine Light
Category: Network Communications

What it means

U0021 means Low Speed CAN Communication Bus (+) Open.

Check these first

An open in the low speed CAN positive wire
A broken or corroded low speed CAN positive connector pin
A module that has lost its low speed CAN positive connection
Harness damage that cut the low speed CAN positive conductor

What to do next

Read the code and check module communication status on a scan tool
Test the low speed CAN positive wire for continuity end to end
Check the low speed CAN positive connector pins for corrosion or backing out
Measure bus resistance across the low speed CAN terminals
Repair the open in the low speed CAN positive circuit
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.