What it means
A low signal fault has been detected on the negative line of the medium-speed CAN communication bus. The affected functions depend on the vehicle network layout.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
The medium-speed CAN bus links modules on its network, and CAN-L is the negative signal wire in its paired circuit. U0013 sets when CAN-L is detected abnormally low, often because it is shorted to ground or a water-affected connector is pulling the line down.
Common causes
Most likely first.
- CAN-Low shorted to ground
- Water ingress in door connector
- Gateway module routing CAN messages incorrectly
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 data and communication codes to identify which medium-speed nodes were affected when CAN-L went low.
- Inspect door and body connectors on the affected MS-CAN branch for water ingress, corrosion, or conductive contamination, along with visible CAN-L harness damage.
- With the vehicle powered down as specified by the manufacturer, check CAN-L for an unintended connection to ground and compare with the manufacturer's specification.
- Check connector terminals for moisture bridging the circuit and inspect harness areas where CAN-L may be pinched against body metal.
- Use the manufacturer's network isolation procedure to identify a module or branch that pulls CAN-L low.
- Assess gateway routing after the CAN-L circuit, affected connectors, and module transceivers have been checked.
Common fixes
- Dry and reseal a water-affected door connector, then repair corroded terminals.
- Repair CAN-L wiring shorted to body ground.
- Replace damaged connector seals that allowed water into the MS-CAN connection.
- Replace a module only if isolation confirms its transceiver is pulling CAN-L low.
- Correct gateway routing or configuration if the physical network checks out.
Can I keep driving?
If the affected functions are limited to body or comfort systems and the vehicle remains fully controllable, limit driving and do not depend on those functions. Stop if safety-related systems are affected, since a low CAN-L fault can take multiple modules offline.
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.