What does it mean?
U3003 is a Network (CAN bus, module communications) fault code. It indicates: Battery Voltage . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- Battery warning lamp on, often together with unrelated warning lamps
- Slow cranking or a no-start, especially in cold weather
- Dim lights, slow wipers, displays resetting
- Several modules storing communication and low-voltage codes
- Start-stop function disabled
- Electrical faults that come and go with engine speed
Likely causes
- Weak, sulphated or aged battery that cannot hold voltage under load
- Charging system fault — worn alternator, slipping or glazed belt, failed regulator
- Corroded or loose battery terminals and ground straps
- High resistance in the main power feed or a corroded fuse box contact
- Parasitic drain flattening the battery overnight
- Overcharging from a faulty regulator, taking the voltage above the module limit
- New battery fitted but never registered on vehicles that require it
How to fix it
- Load-test the battery and check its state of charge
- Measure charging voltage at idle and at raised engine speed, and check for excessive ripple
- Clean and tighten the battery terminals, the engine ground strap and the body ground points
- Do a voltage drop test on the main positive and negative cables while cranking
- Measure the parasitic drain once the vehicle has gone to sleep
- Register a replacement battery where the manufacturer requires it
- Clear all codes and retest after the supply is corrected; many of the other codes will not return