What it means
Fault on the combined ACC/ON/START ignition input line of the control module. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
The module monitors a combined ignition input that represents accessory, on, and start states. U300B sets when that ACC/ON/START input circuit has a fault, preventing the module from reliably recognizing the requested ignition state.
Common causes
Most likely first.
- Wiring or connector fault
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules and save freeze-frame or event data to identify the reporting module and note any accompanying ignition input or communication codes.
- Compare the module’s ACC/ON/START live data with the actual switch position, including while turning the key or operating the start control.
- Inspect the named module’s ignition input connector and wiring for damage, loose terminals, corrosion, or an open circuit.
- Test the ACC/ON/START input circuit through each switch position and compare its signals with the manufacturer’s wiring information and specification.
- If the input circuit behaves correctly but communication faults remain, check gateway routing and CAN network operation using the manufacturer’s procedure.
Common fixes
- Repair an open, damaged, or poorly connected ACC/ON/START input wire or connector.
- Replace a faulty ignition switch if its combined input signals fail the specified checks.
- Repair the gateway fault if diagnostics confirm that it is routing the module’s CAN messages incorrectly.
- Repair a confirmed CAN network fault, including a terminating resistor outside the manufacturer’s specification.
Can I keep driving?
Do not rely on the vehicle for a trip if it fails to recognize ignition or start requests, shuts down, or enters limp mode. The fault can interrupt functions that depend on the module’s ignition-state data.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.