What it means
An open fault has been detected on the positive 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 connects modules that exchange vehicle data over CAN-High (+) and CAN-Low (-). U0020 sets when the CAN-High line on this low-speed network is detected as open, interrupting messages to some connected functions.
Common causes
Most likely first.
- CAN-High 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
- Record freeze-frame data and communication codes from all modules, then identify which low-speed CAN modules are not responding.
- Compare the missing modules with the affected vehicle functions to locate the likely low-speed network branch.
- Inspect the low-speed CAN-High wire, body harness connectors, and splices for a break, corrosion, or loose terminal.
- With power removed according to the manufacturer's procedure, check CAN-High continuity through the affected branch against the manufacturer's specification.
- Check low-speed network termination and gateway routing using the vehicle's wiring information after confirming the wire path.
Common fixes
- Repair an open or damaged low-speed CAN-High wire.
- Clean, secure, or replace damaged CAN-High connector terminals or splices.
- Replace a faulty terminating resistor only if testing confirms it is outside specification.
- Correct a confirmed gateway routing fault using the manufacturer's diagnostic procedure.
Can I keep driving?
If the missing low-speed network functions include safety equipment, lighting, or controls, avoid driving until those functions are restored. Otherwise, restrict driving to what is necessary and arrange diagnosis, since additional functions may be lost as communication deteriorates.
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.