Network, generic (extended)

U300C Ignition Input Off/On/Start

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U300C covers an ignition input that, going by the title, is meant to distinguish Off, On and Start rather than simply switching between live and dead. GM's 2012 light-duty truck wiring manual has exactly such a circuit, labelled Off/Run/Crank: unlike the plain accessory and run/crank feeds it takes separate states for key out or accessory, key in and off, run, and crank. A module reading a coded line of this kind sets U300C when the state it decodes is invalid or disagrees with the power mode on the network. No manufacturer documents U300C publicly. Because one wire carries several states, a slight resistance in the switch or a poor connector can shift the level into the wrong state without breaking the circuit, so measuring the line in every key position beats a simple continuity test.

Likely causes

Contact resistance in the ignition switch shifting the coded level
Corroded or loose pin at the switch or module connector
Input wire open or shorted to ground or voltage
Faulty input stage in the reading module

Symptoms

  • Vehicle does not switch power modes as expected, e.g. no start or no shutdown
  • Intermittent warnings that depend on key position or switch movement

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€30–500
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ignition inputignition switchignition wiring

Related codes

U3011U300DU3010U300AP2533

References: law.cornell.edu, gmupfitter.com.