U300CIgnition Input Off/On/Start
- Generic OBD-II code · Network
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyContact resistance in the ignition switch shifting the coded level
- PossibleCorroded or loose pin at the switch or module connector
- PossibleInput wire open or shorted to ground or voltage
- RareFaulty input stage in the reading module
- Repair
- Medium · Usually a job for a workshop.Typical cost: €30 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.