U302B
Ignition Output Accessory/On/Start.
- Generic
- Network
- Generic code
- Medium severity
How this code is built
- U
- SystemNetwork
- 3
- Defined bySAE, the same on every car
- 0
- AreaNetwork area 0
- 2B
- FaultThe specific fault in that area
What it means
U302B is a generic communication-category code linked to the ignition output signal for the accessory, on (run) and start positions. A control module has detected that this ignition status signal is missing, implausible or does not agree with what it expects. The exact trigger and which module sets it vary by vehicle, so use the manufacturer's wiring information for the model.
Symptoms
- Engine slow to start, fails to crank or starts intermittently
- Accessories such as the radio or windows work intermittently or not in certain key positions
- Several warning lights on the dashboard, sometimes with other modules reporting faults
- Ignition position not recognised, or the vehicle seems to think it is still switched on or off
- Flat battery after standing, if a module is not powering down
Possible causes
- Low battery voltage, a weak battery or poor battery terminal or earth connections upsetting the module supplylikely
- Wiring or connector fault (corrosion, chafing, loose pin, high resistance) in the ignition feed or signal circuitlikely
- Worn or faulty ignition switch, or a faulty start/stop or keyless entry module, where fittedpossible
- Blown fuse or faulty relay in the ignition feed circuitpossible
- Fault inside the body control or other ignition-monitoring module, including water ingresspossible
- Outdated or faulty module software, where the manufacturer lists it as a known cause for the modelrare
How to diagnose
- Read all stored and pending codes in every module, and record the freeze frame data and any other U or power supply codes
- Test battery condition and state of charge, then check charging voltage with the engine running
- Check the relevant fuses and relays in the ignition feed circuit, using the vehicle's wiring diagram to identify them
- Inspect the battery terminals, main earth points and the connectors at the ignition switch and affected modules for corrosion, damage or looseness
- Watch live data for ignition status in the modules while turning the key through each position, and look for dropouts or disagreement between modules
- Voltage drop test the ignition feed and earth circuits under load, and wiggle test the wiring while monitoring the signal
- Check for manufacturer service bulletins or software updates relevant to the model and engine
- If the wiring and supplies are good, test or substitute-check the ignition switch or the module suspected, following the manufacturer's procedure
How to fix
- Charge or replace the battery and clean and tighten the terminals and earth connections
- Repair or replace damaged wiring and connectors in the ignition circuit, using proper terminals and sealing
- Replace the faulty fuse or relay after finding out why it failed
- Replace a worn ignition switch or faulty start/stop or keyless module, and carry out any required programming
- Repair or replace a confirmed faulty control module, with correct coding for the vehicle
- Update the module software where an outdated calibration is a known cause for that model
Can I keep driving?
The car may be driveable if it starts and runs normally, but get it checked soon because intermittent ignition signal faults can leave you stranded or cause other systems to misbehave. If it stalls, fails to start reliably or shows safety-related warnings, stop driving and have it inspected.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.