U300B — Ignition Input Accessory/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 U300B 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 U300B does not return.
Approximate repair cost
Repair cost depends on which component actually failed. Typical scenarios for U300B (parts and labor, USD, independent shop rates): Repair coil wiring or connector: $80–$250; Replace an ignition coil: $150–$380; Replace spark plugs: $90–$250. Overall, fixing U300B 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 is the most common cause of U300B?
The most common causes are: faulty ignition control circuit in the ECM, moisture or oil in the spark plug wells, failed ignition coil or coil pack.
What does OBD2 code U300B mean?
Ignition Input Accessory/On/Start (U300B): a vehicle network fault affecting the ignition system. Short, gentle driving to a shop is usually possible. Most often caused by faulty ignition control circuit in the ECM, moisture or oil in the spark plug wells, failed ignition coil or coil pack.
Will U300B clear itself?
Sometimes U300B 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 are the symptoms of U300B?
Drivers most often notice: Check Engine Light on, increased fuel consumption, hard starting, especially in damp weather, loss of power and jerking.
How much does it cost to fix U300B?
Repair coil wiring or connector — $80–$250; Replace an ignition coil — $150–$380; 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.
What does U300B mean?
When U300B appears on a scan, it means the control module has flagged a problem in the ignition system: ignition input accessory/on/start. The fault sits in the vehicle network group of the OBD2 standard.
To set U300B, the control module compares live readings of the ignition system against calibrated limits, usually over several drive cycles. Alongside the code it stores a freeze frame — a snapshot of rpm, load, coolant temperature and vehicle speed at the moment of failure. Reading that snapshot first saves diagnostic time.
Common symptoms of U300B
- Check Engine Light on
- increased fuel consumption
- hard starting, especially in damp weather
- loss of power and jerking
- misfires under load or at idle
What causes U300B?
- faulty ignition control circuit in the ECM
- worn spark plugs or wrong plug gap
- moisture or oil in the spark plug wells
- failed ignition coil or coil pack
- damaged coil wiring or connector
How to diagnose U300B
- Connect an OBD2 scanner, confirm U300B 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 U300B does not return.
How to fix U300B
Start with the cheapest and most common causes before replacing expensive parts:
- Repair coil wiring or connector — $80–$250
- Replace an ignition coil — $150–$380
- Replace spark plugs — $90–$250
How much does it cost to fix U300B?
Repair cost depends on which component actually failed. Typical scenarios for U300B (parts and labor, USD, independent shop rates):
- Repair coil wiring or connector: $80–$250
- Replace an ignition coil: $150–$380
- Replace spark plugs: $90–$250
Overall, fixing U300B 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 U300B?
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.