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DTC codes / U30xx · Network wiring and bus faults / U300D

U300DIgnition Input On/Start

U300D is a generic body/network code indicating that a control module has detected an invalid or missing signal on its ignition input on/start circuit, meaning the module cannot confirm the ignition switch is in the ON or START position. The computer sets this code when the voltage on that input does not match the expected state during key-on or cranking, often due to a wiring fault, a failing ignition switch, or a module power supply issue.

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

Symptoms you may notice

  • The warning light (check engine or service vehicle soon) illuminates, but the vehicle may start and run normally with no other noticeable issues.
  • Intermittent no-crank or no-start condition, especially in cold weather or after the vehicle sits for extended periods.
  • The engine may crank but not start, or start and then stall immediately after the key is released from START.
  • Accessory or ignition-run circuits may not power up correctly, causing the radio, windows, or instrument cluster to behave erratically.
  • Remote start or push-button start systems may fail to engage or may require multiple presses to wake the module.
  • Other unrelated modules may set communication codes (U-codes) because they are not being properly powered or woken up.

Common causes, most likely first

  1. Faulty ignition switch or start button assembly, causing an open or high-resistance contact on the ignition input circuit.
  2. Damaged, corroded, or loose wiring and connectors between the ignition switch and the affected control module.
  3. Blown fuse or faulty relay in the ignition power distribution circuit that feeds the module's ignition input.
  4. Poor ground connection at the module or at the ignition switch, leading to voltage drops that confuse the input detection.
  5. Failed or failing control module that cannot correctly interpret the ignition input signal.
  6. Aftermarket alarm or remote start installation with incorrect wiring splicing into the ignition circuit.
  7. Low battery voltage or a weak battery that drops below the module's threshold during cranking.

How to diagnose and fix it

  1. Verify battery voltage and charge the battery if it is below 12.4 volts; a weak battery is a common cause of false ignition input codes.
  2. Check all fuses and relays related to the ignition switch and the affected module, replacing any that are blown or suspect.
  3. Inspect the ignition switch and its connector for corrosion, loose pins, or mechanical wear; test the switch with a multimeter for continuity in ON and START positions.
  4. Perform a voltage drop test on the ignition input circuit at the module connector while cycling the key to confirm the signal reaches the module correctly.
  5. Check and clean all ground connections for the module and the ignition switch, ensuring less than 0.1 volt drop under load.
  6. If wiring and switch test good, substitute a known-good module or reflash the existing module with the latest software to rule out internal faults.
  7. Clear the code and perform a road test, including multiple key cycles and a cold start, to confirm the repair and ensure the code does not return.

Ignition Input Faults That Fool Technicians

  • A weak or failing battery is one of the most overlooked causes of U300D; the voltage sag during cranking can drop the ignition input signal below the module's threshold, setting the code without any wiring fault.
  • Aftermarket remote start or alarm systems frequently back-probe or splice into the ignition input wire, introducing resistance or intermittent opens that mimic a failing ignition switch.
  • This code does not always mean the ignition switch is bad; a poor ground at the module can cause the same symptom because the module references the ignition input against ground, and a lifted ground skews that reference.
  • On vehicles with push-button start, the start button itself can develop high contact resistance over time, and the code may only set intermittently when the button is pressed at an angle.
  • U300D can be stored alongside communication codes (U-codes) in other modules because those modules may not receive the proper ignition wake-up signal, leading technicians to chase network faults instead of the root ignition input issue.

Can I keep driving with U300D?

If the vehicle starts and runs normally, you can drive it to a repair shop, but avoid long trips or situations where a no-start would be dangerous. If the vehicle is intermittently failing to crank or start, do not drive it until the fault is repaired, as it may leave you stranded.

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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