U300C code meaning
Ignition Input Off/On/Start
What U300C means
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.
U300C at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U300C
- 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
Common causes of U300C
- Most 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
- Less likelyFaulty input stage in the reading module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Ignition inputIgnition switchIgnition wiring
What each part of U300C means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 0C | The exact fault in that area. |
What to do next
- Note U300C and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U300C
What does U300C mean?
U300C means "Ignition Input Off/On/Start". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U300C?
U300C does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U300C fail an emissions test?
U300C is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U300C to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U300C clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.