What it means
An open 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 uses CAN-H and CAN-L as a paired signal circuit to link modules on that network. U0012 specifically indicates an open in CAN-L, which can interrupt communication on the affected MS-CAN branch and make connected modules disappear.
Common causes
Most likely first.
- CAN-Low wire broken in body harness
- Loose body control module 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 CAN nodes lost contact when the code set.
- Check whether the missing modules share a branch, then inspect that branch's CAN-L wire and body control module connectors for looseness or visible damage.
- With the vehicle powered down as specified by the manufacturer, check continuity along the CAN-L path and compare with the manufacturer's specification.
- Inspect harness routing through body areas and connectors for a broken conductor, pulled terminal, or previous repair at an open point.
- Use the manufacturer's branch isolation procedure to locate the section where CAN-L continuity is lost.
- Check gateway routing only after the CAN-L path is confirmed continuous and connector terminals are sound.
Common fixes
- Repair a broken CAN-L conductor in the body harness.
- Reseat and secure a loose body control module connector.
- Repair a pulled-back or damaged CAN-L terminal.
- Replace a damaged harness section where the conductor cannot be reliably repaired.
- Correct gateway routing if the CAN-L circuit is continuous and module connections are sound.
Can I keep driving?
If essential controls remain normal and only body or comfort functions are affected, cautious short-distance driving may be possible, but avoid relying on those functions. Do not drive if the affected network includes safety-critical systems or warning indications you cannot assess.
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.