What it means
A low signal fault has been detected on the positive 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 network sends messages between vehicle modules over CAN-High (+) and CAN-Low (-). U0018 is set when the CAN-High line is detected at an abnormally low signal level, consistent with a short to ground or another fault pulling the line down.
Common causes
Most likely first.
- CAN-High shorted to ground
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
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
- Record freeze-frame data and all module communication codes with a scan tool, noting which medium-speed CAN modules are offline.
- Check whether the network fault coincides with inoperative body functions or appeared after wiring or accessory work.
- Inspect the CAN-High wire and connectors for chafing, crushed insulation, corrosion, or contact with ground.
- Following the manufacturer's power-down procedure, isolate network sections and test CAN-High for an unintended connection to ground.
- Check termination and gateway routing against the vehicle wiring information after ruling out a CAN-High short to ground.
Common fixes
- Repair chafed or crushed CAN-High wiring that is contacting ground.
- Restore damaged or corroded connector terminals and secure the harness away from grounded edges.
- Replace a terminating resistor only if testing shows it is outside the manufacturer's specification.
- Correct a confirmed gateway routing fault using the manufacturer's procedure.
Can I keep driving?
Avoid driving if warnings or failed functions affect steering, braking, restraints, lighting, or other safety systems; the network fault can make those functions unavailable. If the vehicle remains controllable, limit driving and have the CAN-High short diagnosed promptly.
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.