U300E code meaning
Ignition Input On
What U300E means
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.
U300E 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 U300E
- Functions on the run-only feed dead with the engine running
- Code logged right after each start with no other complaint
Common causes of U300E
- PossibleBlown fuse or faulty run relay on the run-only feed
- PossibleInput wire open, shorted or backfed
- PossibleModule software misjudging the intended drop during cranking
- Less likelyWorn run contact in the ignition switch
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Ignition inputIgnition relayIgnition wiring
What each part of U300E means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 0E | The exact fault in that area. |
What to do next
- Note U300E 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 U300E
What does U300E mean?
U300E means "Ignition Input On". U300E deals with an ignition input that should be live only in On, without Start. It is a generic code, so it means the same on every make.
Is it safe to drive with U300E?
U300E does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U300E fail an emissions test?
U300E 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 U300E 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 U300E clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.