U300E Ignition Input On

U300E deals with an ignition input that should be live only in On, without Start. Such run-only feeds exist because many loads are shed while the starter turns; GM's 2012 truck wiring manual, for instance, keeps its retained accessory power relay energized in all modes but two, crank and off-awake. If a module sees this run-only line high in Off or Accessory, or low while the network reports run, it stores U300E. A brief drop during cranking is intended and must not be judged as a fault, so a module that flags the code right after every start points more to a timing or software issue than to wiring. No manufacturer documents U300E publicly. A real wiring fault usually leaves functions on that feed dead with the engine running, which helps to separate the two cases.

Likely causes

Symptoms

• Functions on the run-only feed dead with the engine running
• Code logged right after each start with no other complaint

Repair estimate

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

Parts involved

Related codes

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