What it means
A low signal 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 carries messages between vehicle modules over CAN-High (+) and CAN-Low (-). U0021 means the CAN-High line on this network is being detected at a low signal level, often because it is shorted to ground.
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
- Capture freeze-frame data and all module communication codes, noting which low-speed CAN modules are affected.
- Check whether the failed functions share the low-speed network and whether the fault followed recent wiring work.
- Inspect low-speed CAN-High wiring and connectors for abrasion, crushing, corrosion, or contact with ground.
- Follow the manufacturer's power-down procedure, then test CAN-High for a short to ground and isolate network sections to narrow the fault.
- Check the low-speed termination and gateway routing against vehicle wiring information after ruling out a CAN-High ground short.
Common fixes
- Repair low-speed CAN-High wiring that is shorted to ground.
- Repair corroded or damaged connector terminals and protect the harness from the grounding point.
- Replace the terminating resistor only if tests show it is outside the manufacturer's specification.
- Correct a confirmed gateway routing fault using the prescribed diagnostic procedure.
Can I keep driving?
Do not drive if affected functions compromise braking, steering, restraints, lighting, or other safety systems. If the vehicle remains safe to control, minimize driving and arrange prompt repair because the low-speed network may lose additional functions.
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.