What are the common causes of U300D?

A failing ignition switch or ignition module. An open or short in the ignition input circuit. Corrosion or a loose connection in the ignition circuit. A blown fuse feeding the ignition input. A failing ECM or PCM reading the ignition input.

Check the fuse feeding the ignition input circuit. Inspect the ignition switch and ignition input wiring. Test the ignition input circuit for opens and shorts. Read the ignition input state with a scan tool while turning the key. Replace the ignition switch or module if the input does not change. Clear the code and retest.

U300D

Ignition Input On/Start

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

What it means

U300D means Ignition Input On/Start.

Check these first

A failing ignition switch or ignition module
An open or short in the ignition input circuit
Corrosion or a loose connection in the ignition circuit
A blown fuse feeding the ignition input
A failing ECM or PCM reading the ignition input

What to do next

Check the fuse feeding the ignition input circuit
Inspect the ignition switch and ignition input wiring
Test the ignition input circuit for opens and shorts
Read the ignition input state with a scan tool while turning the key
Replace the ignition switch or module if the input does not change
Clear the code and retest

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Updated

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