What does it mean?
U1121 is a Network (CAN bus, module communications) fault code. It indicates: Lost Communication With Power Management Battery Voltage .
Symptoms
- Battery or charging warning lamp, or a message about the electrical system
- Charging voltage jumping around or staying at one fixed value
- Stop-start function disabled and the engine no longer shutting down at rest
- Comfort loads such as seat heating or the audio system being cut back at idle
- Short battery life, repeated flat batteries or unexplained drain
- The state of charge value in live data is missing or clearly wrong
Likely causes
- Loose, corroded or overtightened battery sensor clamp on the negative post
- Broken or chafed pigtail wire from the sensor to the harness, a very common failure point
- Open or shorted data line between the battery sensor and the module that manages electrical loads
- New battery fitted without carrying out the battery registration or reset procedure
- Internal failure of the battery monitoring sensor
- Poor main ground connection or a heavily corroded battery terminal
- Body control module fault or outdated software
How to fix it
- Inspect the sensor clamp for corrosion, looseness or damage, and check the pigtail where it flexes at the sensor body
- Clean and correctly tighten the battery terminals, then retest
- Measure supply voltage and check the data line from the sensor to the receiving module for continuity and shorts
- Read the battery sensor live data and compare its readings with an external meter and clamp
- Test the battery and alternator, because a failing battery makes the sensor report implausible values
- If a battery was fitted recently, carry out the battery registration or reset procedure and confirm the correct type and capacity were entered
- Replace the battery monitoring sensor only after wiring, terminals and registration have been checked