U300BIgnition Input Accessory/On/Start
- Generic OBD-II code · Network
- How the car detects it
- 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.
- What you may notice
- Functions on the accessory and run feed missing or behaving oddly
- Affected module running in a fallback mode with a warning
- Likely causes
- 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
- RareBody control module sending a wrong power mode on the bus
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.