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DTC codes / U30xx · Network wiring and bus faults / U300B

U300BIgnition Input Accessory/On/Start

U300B is a generic body/control-module code indicating that a module has detected an implausible or missing signal on the ignition input circuit that reports Accessory, On, and Start positions. The computer compares the expected voltage states from the ignition switch or push-button start system against what it actually sees, and sets this code when the inputs do not match a valid key-on sequence.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork wiring and bus faults

Symptoms you may notice

  • The warning light may be the only thing the driver notices, with no change in how the vehicle runs or starts.
  • Intermittent no-crank or no-start conditions, especially when the ignition is cycled quickly.
  • Accessory power (radio, windows, blower) staying on or failing to come on when the key or button is moved to the corresponding position.
  • The instrument cluster failing to wake up or displaying incorrect ignition status.
  • Random warning lights or communication codes from multiple modules that share the ignition status signal.
  • The push-button start requiring multiple presses or a longer press than normal to engage the starter.

Common causes, most likely first

  1. A failing ignition switch or push-button start module that no longer produces clean Accessory, On, and Start signals.
  2. Worn or corroded wiring and connectors in the ignition input circuit, particularly at the steering column or start button.
  3. A weak or failing vehicle battery or poor battery terminal connection causing voltage drops that confuse the ignition state detection.
  4. A faulty body control module (BCM) or gateway module that is misreading or failing to pass along the ignition status.
  5. Blown fuses or damaged relays in the ignition switch feed circuit.
  6. Aftermarket accessories or remote start systems improperly tied into the ignition wiring and back-feeding voltage.
  7. Water intrusion or chafed harnesses in the dash or kick-panel area affecting the ignition input signals.

How to diagnose and fix it

  1. Verify battery condition and terminal cleanliness first, since low voltage is a common trigger for false ignition input codes.
  2. Scan all modules for related codes and record freeze-frame data to see which module set U300B and under what conditions.
  3. Inspect the ignition switch or push-button start connector for corrosion, loose pins, and evidence of overheating.
  4. Back-probe the ignition input wires at the module that set the code and compare Accessory, On, and Start voltages against the service information.
  5. Check all fuses and relays that feed the ignition switch circuit, and test for voltage drops across the switch contacts.
  6. If wiring and power supply check out, substitute a known-good ignition switch or start button and re-test the circuit.
  7. Clear the code and perform a road test with the scanner recording ignition status parameters to confirm the repair.

Ignition status faults that fool good technicians

  • A weak battery that still starts the engine can cause U300B because the voltage dip during cranking makes the ignition input signal drop below the module's valid threshold.
  • Aftermarket remote starters and alarm systems are a frequent culprit because they splice into the ignition wires and can back-feed voltage that the module reads as an invalid key position.
  • The code often sets in a module other than the one that actually has the wiring fault, so chasing the module that stored U300B without checking shared ignition feeds leads to unnecessary module replacement.
  • On push-button vehicles, a failing brake pedal switch or clutch switch can prevent the Start signal from being recognized, and the module may log U300B instead of a dedicated switch code.
  • Corrosion inside the ignition switch itself can create intermittent opens that only appear when the key is wiggled or when cabin temperature changes, making the code hard to reproduce in the shop.

Can I keep driving with U300B?

The vehicle may start and run normally, but an ignition input fault can cause an unexpected no-start or accessory power loss, so avoid long trips and do not shut the engine off in an unsafe location until the cause is found. If the car fails to crank or the dash goes dead intermittently, stop driving and have it diagnosed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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

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