U300D Ignition Input On/Start
What does U300D mean?
U300D — Ignition Input On/Start. This is an OBD-II diagnostic trouble code a scanner retrieves when your vehicle's computer detects ignition input on/start. The level of risk depends on how the vehicle is currently running and how long you've been driving with the light on.
Common causes of U300D
- Shorted or open data bus wiring between modules
- Failed module interrupting the shared network
- Corroded, loose, or water-damaged connectors
- Low system voltage during module communication
- Faulty module power supply or ground
How to fix U300D
These are general steps for this system. The exact cause depends on your vehicle — confirm with live scan data before replacing parts.
- Scan all modules to identify which have lost communication
- Measure data bus resistance and voltages at the diagnostic connector
- Check fuses, relays, and grounds powering the affected module
- Isolate modules on the bus to find the one causing the dropout
- Repair wiring or replace the failed module after confirming power and bus health