U300E: Ignition Input On

What it means

Fault on the ignition ON input — module does not see ON state when other inputs say ON, or vice versa.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

The module monitors its ignition ON input to determine whether the vehicle is meant to be powered for normal operation. U300E sets when the ON signal disagrees with other inputs or the actual ignition state, so the module may misjudge whether the vehicle is on.

Common causes

Most likely first.

1. Ignition switch contacts worn
2. Gateway module routing CAN messages incorrectly
3. 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

1. Scan all modules and save freeze-frame or event data, then note whether the code occurs with the ignition off, on, or during starting.
2. Compare ignition ON live data with the actual switch position and look for a missing or unexpected ON state.
3. Inspect the ignition switch connector and the ON input wiring for corrosion, loose terminals, or damage.
4. Test the ON input signal at the module and switch, comparing results with the manufacturer’s wiring information and specification.
5. If the ON signal is correct at the module, check for gateway or CAN faults that could report a different ignition state.

Common fixes

• Repair a loose, corroded, or damaged ignition ON input connection or wire.
• Replace the ignition switch if its ON contact fails the specified circuit checks.
• Repair the gateway or CAN network fault if diagnostics show incorrect state communication.

Can I keep driving?

Avoid driving if the vehicle unexpectedly loses power, fails to stay on, or disables an essential function. An unreliable ON signal can cause dependent modules to shut down or behave intermittently.