Network, generic (extended)

U300F Ignition Input Accessory

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U300F refers to the accessory input, the line that powers comfort loads such as audio or power windows when the key is in accessory. In GM's 2012 pickup wiring manual this accessory voltage circuit is active in accessory and run but switched off in crank and in both off positions. A module stores U300F when that pattern breaks: voltage on the accessory input with the key out, or none while the body controller broadcasts accessory or run. No manufacturer documents U300F publicly. Since the accessory feed is often routed through a relay and shared by many consumers, a stuck relay can leave radio and windows working with the key removed and slowly flatten the battery, whereas an open feed simply kills those functions; checking which of the two applies is the first step.

Likely causes

Accessory relay sticking closed or not pulling in
Blown accessory fuse or open feed wire
Worn accessory contact in the ignition switch
Aftermarket equipment backfeeding the accessory line

Symptoms

  • Radio, windows or sockets dead in accessory, or still on with the key out
  • Battery drained after parking when the feed stays live

Repair estimate

DifficultyEasyDIY possible
Parts and labour€10–350
Time0.3–2 h

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

Parts involved

ignition inputignition switchignition wiring

Related codes

U3004U3005U300BU3022U300E

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