What it means
Fault on the ON/START ignition input line — module sees inconsistent ON or START signal. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
The module monitors an ignition input used to recognize the on and start states. U300D sets when the ON/START signal is missing, contradictory, or present at the wrong time, leaving the module unable to distinguish a normal on state from a start request.
Common causes
Most likely first.
- Wiring or connector fault
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules and save freeze-frame or event data, noting whether the code appeared during cranking or while the ignition was on.
- Watch the module’s ON/START live data with the ignition on and while cranking, and compare the displayed states with the actual switch position.
- Inspect the ON/START input wiring and connector for loose pins, corrosion, wear, or damage.
- Test the ON and START portions of the input circuit separately against the manufacturer’s wiring information and specification.
- If both signals reach the module correctly, investigate related communication codes and check gateway routing or CAN operation.
Common fixes
- Repair a damaged or poorly connected ON/START input wire or connector.
- Replace a worn ignition switch if the ON or START contact fails its circuit check.
- Repair the gateway or CAN network fault if testing confirms incorrect ignition-state messaging.
Can I keep driving?
Avoid driving if the vehicle stalls, fails to start reliably, or loses an essential function when the ignition state changes. A faulty ON/START input can interrupt dependent systems or prevent a dependable restart.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.