Network, generic (extended)

U300B Ignition Input Accessory/On/Start

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Blown fuse or faulty relay in the switched feed to this input
Input wire open, shorted or backfed from another circuit
Worn ignition switch contact for this position group
Body control module sending a wrong power mode on the bus

Symptoms

  • Functions on the accessory and run feed missing or behaving oddly
  • Affected module running in a fallback mode with a warning

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–500
Time0.5–2.5 h

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

Parts involved

ignition inputignition switchignition wiring

Related codes

U300FU300DU3022U3004U300A

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