What it means
Fault on the ignition ACC (accessory) input. 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 the ignition accessory input to recognize when accessory power is requested. U300F sets when the ACC signal is missing or inconsistent with the switch state, which can affect functions that depend on accessory mode.
Common causes
Most likely first.
- Ignition switch contacts worn
- 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, then identify which module reports the ACC input fault.
- Compare the module’s ACC live data with the actual accessory switch position and check whether the state changes consistently.
- Inspect the ACC input connector and wiring for loose terminals, corrosion, or physical damage.
- Test the accessory input signal at the switch and module against the manufacturer’s wiring information and specification.
- If the ACC circuit is sound, investigate associated communication codes and check gateway routing or CAN operation.
Common fixes
- Repair a damaged, corroded, or loose ACC input wire or connector.
- Replace a faulty ignition switch if its accessory contact fails testing.
- Repair the gateway or CAN network fault if incorrect accessory-state messages are confirmed.
Can I keep driving?
Driving is generally not affected if the fault is limited to accessory functions, but do not drive if essential systems also malfunction or the vehicle shuts down. Accessory features may fail or switch off unexpectedly.
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.