What are the common causes of U0099?

A short to voltage on the bus H (-) wire. A module on bus H driving the (-) line above its normal level. A bad ground forcing the bus H (-) reference high. An added accessory backfeeding the bus H (-) line.

Read stored codes on every module that shares bus H. Measure the bus H (-) line for a short to voltage. Check the ground points shared by the modules on bus H. Disconnect added accessories one at a time to rule out a backfeed. Replace the module driving the bus H (-) line high. Clear the code and retest.

U0099

Vehicle Communication Bus H (-) High

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

What it means

U0099 means Vehicle Communication Bus H (-) High.

Check these first

A short to voltage on the bus H (-) wire
A module on bus H driving the (-) line above its normal level
A bad ground forcing the bus H (-) reference high
An added accessory backfeeding the bus H (-) line

What to do next

Read stored codes on every module that shares bus H
Measure the bus H (-) line for a short to voltage
Check the ground points shared by the modules on bus H
Disconnect added accessories one at a time to rule out a backfeed
Replace the module driving the bus H (-) line high
Clear the code and retest

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Updated

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