U0021: Low-Speed CAN Bus Open Circuit
The U0021 code indicates an open circuit condition in the low-speed Controller Area Network (CAN) bus, also known as CAN-B or comfort/convenience CAN. This bus operates at 125 kbps and connects non-safety-critical modules such as climate control, infotainment, seat, and door modules. An open circuit means the control module has detected a break in continuity on the CAN-H and/or CAN-L lines, preventing bidirectional communication between bus nodes.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Open or out-of-spec termination resistor on the low-speed CAN bus (should be 120 ohms at each end, 60 ohms in parallel)
- Cut, corroded, or poorly connected CAN-H or CAN-L wiring at splices or connectors
- Internal failure of a control module causing an open on the bus
- Moisture or corrosion in connectors of low-speed bus modules
- Incorrect previous electrical repair or modification
3. Symptoms You Will Notice
- Malfunction of comfort systems (e.g., power windows, locks, climate control)
- Illuminated check engine light or communication-related warning messages
4. Step-by-Step Diagnosis
Verify bus termination resistance (120 ohms at each end, 60 ohms measured across the bus)
Check continuity and condition of CAN-H and CAN-L wiring, especially at connectors and splices
Inspect and test low-speed bus modules for internal faults or corrosion
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive short distances as the low-speed CAN bus controls comfort features, not critical safety systems. However, some non-essential functions may be inoperative.
How much does it cost to fix?▼
Repair costs typically range from $100 to $600, depending on whether the issue is a simple wiring repair or requires module replacement.