A faulty ignition switch or key cylinder that fails to make clean contact in one or more positions.; Worn, corroded, or loose wiring and connectors on the ignition sense circuit between the switch and the affected module.; A blown fuse or poor ground connection that interrupts power or reference voltage to the ignition input circuit.; A failing body control module (BCM) or other module that supplies or interprets the ignition status signal.; Voltage drops or spikes from a weak battery or failing alternator that confuse the module's ignition detection logic.; Aftermarket accessories or remote start systems tapped into the ignition wiring and disturbing the signal.; Internal fault in the module that stores the code, causing it to misread an otherwise correct ignition input.

DTC codes / U30xx · Network wiring and bus faults / U300C

U300CIgnition Input Off/On/Start

Q: What does code U300C mean?

Ignition Input Off/On/Start. U300C is a generic body/network code indicating that a control module has detected an invalid or missing signal on the ignition input circuit, meaning it cannot reliably determine whether the ignition is off, on, or in the start position. The module logs this code when the voltage pattern on the ignition sense wire does not match the expected sequence during a key cycle or when the signal is absent entirely.

U300C is a generic body/network code indicating that a control module has detected an invalid or missing signal on the ignition input circuit, meaning it cannot reliably determine whether the ignition is off, on, or in the start position. The module logs this code when the voltage pattern on the ignition sense wire does not match the expected sequence during a key cycle or when the signal is absent entirely.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork wiring and bus faults

Symptoms you may notice

The warning light may be the only sign, with no noticeable change in how the vehicle runs or drives.
The engine may crank but fail to start because a module does not see the ignition-on signal.
Intermittent no-start or stalling conditions that come and go with vibration or temperature changes.
Accessories or dashboard displays may behave erratically, such as staying on after the key is removed or not powering up with the key.
A no-crank condition where the starter does not engage when the key is turned.
Multiple unrelated warning lights or communication codes may appear because several modules share the ignition signal.

Common causes, most likely first

A faulty ignition switch or key cylinder that fails to make clean contact in one or more positions.
Worn, corroded, or loose wiring and connectors on the ignition sense circuit between the switch and the affected module.
A blown fuse or poor ground connection that interrupts power or reference voltage to the ignition input circuit.
A failing body control module (BCM) or other module that supplies or interprets the ignition status signal.
Voltage drops or spikes from a weak battery or failing alternator that confuse the module's ignition detection logic.
Aftermarket accessories or remote start systems tapped into the ignition wiring and disturbing the signal.
Internal fault in the module that stores the code, causing it to misread an otherwise correct ignition input.

How to diagnose and fix it

Verify battery voltage and condition, then clear the code and perform a key cycle to see if it returns immediately.
Inspect the ignition switch and key cylinder for smooth operation and check for any aftermarket wiring spliced into the ignition circuit.
Check all fuses and grounds related to the ignition switch and the module that set the code, cleaning and tightening connections as needed.
Use a scan tool to view live data for ignition switch status and compare it with actual key position through off, on, and start.
Back-probe the ignition sense wire at the module and measure voltage in each key position, looking for drops, opens, or incorrect voltages.
If the signal is correct at the module but the code persists, test or substitute the module that stores the code, following manufacturer procedures for programming.
Repair or replace the faulty ignition switch, wiring harness, or module as indicated by the test results, then clear codes and verify a complete drive cycle.

Ignition Switch Signal Traps to Check First

A common oversight is assuming the ignition switch is bad when the real problem is a corroded connector or chafed wire in the harness, so always load-test the circuit before replacing parts.
Aftermarket remote starters and alarm systems frequently back-probe or splice the ignition wire, and a poor connection there can set U300C even though the factory wiring is intact.
Some vehicles set U300C when the battery is disconnected or deeply discharged during service, and the code may be a history code that does not indicate a current fault.
The code can be stored by a module that is not the one directly wired to the ignition switch, so checking which module set the code is essential before tracing the circuit.
A failing body control module can report an ignition input fault even when the switch and wiring test good, because the BCM interprets and distributes the ignition status to other modules.

Can I keep driving with U300C?

If the vehicle starts and runs normally, driving to a repair shop is acceptable, but avoid long trips and be prepared for a no-start condition. If the engine will not start or stalls while driving, do not continue driving and have the vehicle towed to prevent safety risks.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.