What it means
An open fault has been detected on the negative line of the low-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 low-speed CAN network carries data over paired CAN-High (+) and CAN-Low (-) wires. U0025 is set when the CAN-Low line is detected as open, interrupting or degrading messages on the low-speed network.
Common causes
Most likely first.
- CAN-Low wire broken
- 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
- Capture freeze-frame data and all module communication codes, then identify which low-speed CAN modules are not responding.
- Compare the affected functions with the low-speed network layout to find the likely CAN-Low branch.
- Inspect the CAN-Low wire, connectors, and splices for a broken conductor, loose terminal, corrosion, or harness damage.
- Following the manufacturer's power-down procedure, test CAN-Low continuity across the affected branch against the manufacturer's specification.
- Verify low-speed network termination and gateway routing with the wiring information once the CAN-Low path has been checked.
Common fixes
- Repair a broken or damaged low-speed CAN-Low wire.
- Clean, secure, or replace corroded or loose CAN-Low terminals and splices.
- Replace a terminating resistor only if testing confirms it is outside the manufacturer's specification.
- Correct a confirmed gateway routing fault using the prescribed procedure.
Can I keep driving?
If the lost low-speed network functions include safety equipment or vehicle controls, do not drive until they are restored. Otherwise, limit use and arrange diagnosis soon because the open CAN-Low line can interrupt additional module communication.
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.