U300B code meaning
Ignition Input Accessory/On/Start
What U300B means
Judging by its name, U300B concerns the ignition line that should carry voltage in Accessory, On and Start, i.e. in every key position except Off. A module sets it when the level on that wire contradicts the power mode it has been told or sees on its other inputs, for example live with the vehicle off, or dead while the bus reports run. No manufacturer documents U300B publicly. GM's 2012 pickup wiring manual shows the principle: the body control module reads several discrete switch circuits, decides on the power mode and broadcasts it, and modules with their own switched inputs may fall back to a default mode when that message disagrees with their own pins. GM also shows that such lines are cut differently, e.g. its accessory feed drops out while cranking, which this input type should not do.
U300B at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U300B
- Functions on the accessory and run feed missing or behaving oddly
- Affected module running in a fallback mode with a warning
Common causes of U300B
- PossibleBlown fuse or faulty relay in the switched feed to this input
- PossibleInput wire open, shorted or backfed from another circuit
- PossibleWorn ignition switch contact for this position group
- Less likelyBody control module sending a wrong power mode on the bus
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 U300B means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 0B | The exact fault in that area. |
What to do next
- Note U300B 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 U300B
What does U300B mean?
U300B means "Ignition Input Accessory/On/Start". Judging by its name, U300B concerns the ignition line that should carry voltage in Accessory, On and Start, i.e. It is a generic code, so it means the same on every make.
Is it safe to drive with U300B?
U300B does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U300B fail an emissions test?
U300B 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 U300B to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will U300B clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.