U301E

What it means

U301E means a control module has detected an open circuit on its wake-up line, referred to as circuit B. The wake-up wire tells a sleeping module to switch on, for example when the ignition is turned on or a door is opened. The module expected a wake-up signal or a continuous line and saw no continuity or voltage.

Symptoms

• Warning lights on the dash, or a message about a system fault
• One or more systems not working, or working intermittently, such as central locking, lighting or infotainment
• Engine slow to start or failing to start
• Battery draining overnight because a module is not going to sleep properly
• Other communication fault codes stored alongside it
• Fault comes and goes with vibration or temperature

Possible causes

• Damaged, chafed or broken wiring in the wake-up circuit, including an open circuitlikely
• Corroded, loose or pushed-back pins in the module or inline connectorslikely
• Low battery voltage or a poor power or earth supply to the affected modulepossible
• Blown fuse or faulty relay feeding the wake-up line or the modulepossible
• Faulty switch, sensor or controlling module that should supply the wake-up signalpossible
• Internal fault in the module that is reporting the coderare
• Previous repair or accessory fitting that has damaged or interrupted the wiringrare

How to diagnose

1. Read all stored codes and freeze frame data, noting which module reports U301E and whether other communication codes are present.
2. Check battery condition and charging voltage, as low voltage can cause false wake-up and network faults. Fix any problems before going further.
3. Check the fuses and relays for the affected module and the wake-up line using the model wiring diagram.
4. Visually inspect the wiring and connectors for the reporting module, looking for chafing, water ingress, corrosion, aftermarket splices and pushed-back pins.
5. Using the wiring diagram, identify which wire is wake-up circuit B for this model, as the layout varies by make and model.
6. With the connectors disconnected, test the wake-up wire for continuity end to end and for shorts to earth or supply. Wiggle the harness while testing to find intermittent breaks.
7. Check the power and earth supplies at the module, and confirm the signal source such as the switch or controlling module is actually sending the wake-up signal.
8. Clear the codes and retest. If the code returns with sound wiring and supplies, check the manufacturer's information for known module faults or software updates.

How to fix

• Repair or replace the damaged section of wiring, using proper soldered or sealed joints rather than twist-and-tape.
• Clean, re-pin or replace corroded or damaged connectors and terminals.
• Replace the faulty fuse or relay after finding out why it failed.
• Restore a good battery, power and earth supply, and replace the battery if it is weak.
• Replace the faulty switch, sensor or signal-sending module if it proves defective.
• If the reporting module is confirmed faulty, or an update is known to cure the fault, replace the module or update its software, and code it to the vehicle as required.

Can I keep driving?

Nearby codes

• U301DGenericControl Module Wake-up Circuit A Performance
• U301FGenericControl Module Wake-up Circuit B Low