U300A: Ignition Switch

What it means

The control module detects an inconsistent or implausible ignition switch state — multiple positions reported simultaneously, or the position contradicts other inputs.

At a glance

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

How it works

The control module reads the ignition switch state to determine whether the vehicle is off, in accessory or on, or being started. U300A sets when the reported switch position is inconsistent or implausible, such as conflicting positions or a state that disagrees with other inputs.

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 compare the reported ignition state with the actual key or switch position.
2. Watch ignition-state data while moving the switch through off, accessory, on, and start to see whether the reported state jumps, overlaps, or fails to follow.
3. Inspect the ignition switch connector and wiring for loose terminals, wear, corrosion, or damage.
4. Check the switch outputs and related input circuits in each position against the manufacturer’s wiring information and specification.
5. If the switch signals are correct but the module still reports conflicting states, check for gateway message-routing or CAN network faults.

Common fixes

• Repair a loose, corroded, or damaged ignition switch connector or wire.
• Replace a worn ignition switch if its outputs do not match the selected positions.
• Repair the gateway or CAN network fault if diagnostics confirm incorrect state messaging.

Can I keep driving?

Avoid driving if the ignition state changes unexpectedly, the engine or systems shut off, or essential functions become unavailable. An inconsistent switch signal can interrupt vehicle operation or prevent a reliable restart.