U300C — Ignition Input Off/On/Start
vehicle network
Is it safe to drive?
Drive with caution Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.
How to diagnose with an OBD scanner
- Read the code and freeze frame Connect an OBD2 scanner, confirm U300C is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
- Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
- Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
- Subsystem check Swap the coil to another cylinder and see if the fault code follows it.
- Subsystem check Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
- Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U300C does not return.
Approximate repair cost
Repair cost depends on which component actually failed. Typical scenarios for U300C (parts and labor, USD, independent shop rates): Replace an ignition coil: $150–$380; Repair coil wiring or connector: $80–$250; Replace spark plugs: $90–$250. Overall, fixing U300C usually costs between $80 and $380. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.
Common questions
What are the symptoms of U300C?
Drivers most often notice: misfires under load or at idle, loss of power and jerking, Check Engine Light on, hard starting, especially in damp weather.
How much does it cost to fix U300C?
Replace an ignition coil — $150–$380; Repair coil wiring or connector — $80–$250; Replace spark plugs — $90–$250. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.
How do I diagnose U300C myself?
Start with a scan: confirm U300C, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the ignition system, and verify the suspect component against live data before buying parts.
What is the most common cause of U300C?
The most common causes are: worn spark plugs or wrong plug gap, damaged coil wiring or connector, faulty ignition control circuit in the ECM.
Will U300C clear itself?
Sometimes U300C clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.
What does U300C mean?
U300C is logged by the onboard diagnostics when the ignition system does not behave as expected. The official definition is “Ignition Input Off/On/Start”. This is a vehicle network category code.
The module monitors this system continuously while driving. When readings fall outside the expected range for a defined number of drive cycles, it stores U300C and switches on the warning light. Freeze frame data saved with the code shows the exact conditions — rpm, load, temperature, speed — at the moment the fault was detected, which is the best starting point for diagnosis.
What causes U300C?
- damaged coil wiring or connector
- worn spark plugs or wrong plug gap
- faulty ignition control circuit in the ECM
- moisture or oil in the spark plug wells
- failed ignition coil or coil pack
Common symptoms of U300C
- misfires under load or at idle
- loss of power and jerking
- Check Engine Light on
- hard starting, especially in damp weather
- increased fuel consumption
How to diagnose U300C
- Connect an OBD2 scanner, confirm U300C is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
- Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
- Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
- Swap the coil to another cylinder and see if the fault code follows it.
- Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
- Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U300C does not return.
How to fix U300C
Start with the cheapest and most common causes before replacing expensive parts:
- Replace an ignition coil — $150–$380
- Repair coil wiring or connector — $80–$250
- Replace spark plugs — $90–$250
How much does it cost to fix U300C?
Repair cost depends on which component actually failed. Typical scenarios for U300C (parts and labor, USD, independent shop rates):
- Replace an ignition coil: $150–$380
- Repair coil wiring or connector: $80–$250
- Replace spark plugs: $90–$250
Overall, fixing U300C usually costs between $80 and $380. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.
Can you drive with U300C?
Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.