Faulty ignition switch or key cylinder not making proper contact in the accessory position.; Worn or corroded wiring and connectors in the ignition accessory circuit, particularly at the switch or body control module.; Blown fuse or faulty relay that supplies power to the accessory circuit.; Poor ground connection for the module that sets the code, causing a reference voltage error.; Failed body control module (BCM) or other control module that monitors the accessory input.; Aftermarket accessories improperly tapped into the ignition accessory circuit, causing voltage drops or backfeeds.; Water intrusion or physical damage to the wiring harness in the dash or underhood area.

DTC codes / U30xx · Network wiring and bus faults / U300F

U300FIgnition Input Accessory

Q: What does code U300F mean?

Ignition Input Accessory. U300F is a generic body/network code indicating that a control module has detected a fault on the ignition accessory input circuit, which is the voltage signal that tells modules the key is in the accessory position. The computer is not reporting a specific component failure but rather that the expected accessory voltage is missing, out of range, or implausible when compared with other ignition status inputs.

U300F is a generic body/network code indicating that a control module has detected a fault on the ignition accessory input circuit, which is the voltage signal that tells modules the key is in the accessory position. The computer is not reporting a specific component failure but rather that the expected accessory voltage is missing, out of range, or implausible when compared with other ignition status inputs.

SystemNetwork

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaNetwork wiring and bus faults

Symptoms you may notice

Often no symptoms besides the warning light, especially if the fault is intermittent or stored in a module that does not directly control accessory functions.
Accessory-powered devices such as the radio, power outlets, or windshield wipers may not work when the key is turned to accessory.
The vehicle may fail to enter accessory mode, requiring the engine to be started for certain functions to operate.
Intermittent operation of accessories when the key is jiggled or the steering column is moved.
Other unrelated warning lights or communication codes may appear if the fault affects a module on the network.

Common causes, most likely first

Faulty ignition switch or key cylinder not making proper contact in the accessory position.
Worn or corroded wiring and connectors in the ignition accessory circuit, particularly at the switch or body control module.
Blown fuse or faulty relay that supplies power to the accessory circuit.
Poor ground connection for the module that sets the code, causing a reference voltage error.
Failed body control module (BCM) or other control module that monitors the accessory input.
Aftermarket accessories improperly tapped into the ignition accessory circuit, causing voltage drops or backfeeds.
Water intrusion or physical damage to the wiring harness in the dash or underhood area.

How to diagnose and fix it

Verify the battery voltage and condition, as low voltage can cause false accessory input codes.
Check all fuses and relays related to the ignition accessory circuit, replacing any that are blown or suspect.
Inspect the ignition switch for wear and test its continuity in the accessory position using a multimeter.
Disconnect the battery and inspect wiring and connectors from the ignition switch to the body control module for corrosion, looseness, or damage.
Test the accessory input voltage at the module connector with a multimeter while cycling the key to confirm the signal is present and within specification.
If voltage is missing, trace the circuit back to the power source, checking for opens or high resistance; repair as needed.
If all circuits test good, suspect the module itself and follow the manufacturer's diagnostic flow for module replacement or reprogramming.

Accessory circuit faults often misdiagnosed

A common oversight is assuming the ignition switch is bad when the real problem is a corroded connector at the body control module, which can mimic a failed switch.
Aftermarket alarms, remote starters, or phone chargers tapped into the accessory circuit frequently cause U300F by drawing excessive current or backfeeding voltage.
The code can be stored intermittently by a low battery or weak charging system, so always load-test the battery and check charging voltage before replacing parts.
Some vehicles set U300F when the accessory delay relay sticks, keeping the circuit powered after the key is off, which drains the battery and confuses module logic.
U300F does not always mean the accessory circuit is completely dead; a high-resistance connection can cause the module to see an implausible voltage only under certain load conditions.

Can I keep driving with U300F?

You can generally continue driving because the fault primarily affects accessory functions and does not compromise engine operation or safety systems. Avoid using accessory-powered devices until the fault is repaired, and address the code promptly if you notice repeated battery drain or intermittent electrical issues.

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.