What it means
Fault on the OFF/ON/START ignition input line. 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 control module monitors an ignition input that represents off, on, and start states. U300C sets when this OFF/ON/START input circuit does not provide a valid state, so the module may not know whether the vehicle is meant to be off, running, or starting.
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 to identify the reporting module and any related ignition or communication codes.
- Compare the module’s OFF/ON/START live data with the actual switch position as the ignition moves from off through on to start.
- Inspect the OFF/ON/START input wiring and connector at the module for damage, corrosion, loose terminals, or signs of an open circuit.
- Test the input circuit in each relevant switch position and compare its behavior with the manufacturer’s wiring information and specification.
- If the circuit tests correctly, check for gateway routing or CAN faults that could leave the module with incorrect ignition-state information.
Common fixes
- Repair a loose, corroded, or damaged OFF/ON/START input connector or wire.
- Replace a faulty ignition switch if testing shows its output does not follow the selected position.
- Repair the gateway or CAN network fault if the input reaches the module correctly but state messages are still incorrect.
Can I keep driving?
Avoid driving if the vehicle cannot reliably recognize ignition state, shuts down, or loses an essential function. An invalid OFF/ON/START signal can interfere with starting or keep dependent systems from operating correctly.
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.