What are the common causes of U0031?

A short to ground on the Bus A positive wire. A partially shorted Bus A positive connector. A module pulling the Bus A positive voltage low. Water intrusion in the Bus A positive harness.

Read the code and check module communication status on a scan tool. Measure Bus A positive voltage with the key on. Test the Bus A positive wire for a short to ground. Inspect the Bus A positive connectors for moisture. Disconnect modules one at a time to find the one pulling the line low. Clear the code and retest.

U0031

Vehicle Communication Bus A (+) Low

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

What it means

U0031 means Vehicle Communication Bus A (+) Low.

Check these first

A short to ground on the Bus A positive wire
A partially shorted Bus A positive connector
A module pulling the Bus A positive voltage low
Water intrusion in the Bus A positive harness

What to do next

Read the code and check module communication status on a scan tool
Measure Bus A positive voltage with the key on
Test the Bus A positive wire for a short to ground
Inspect the Bus A positive connectors for moisture
Disconnect modules one at a time to find the one pulling the line low
Clear the code and retest

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Updated

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