U0020
Low Speed CAN Communication Bus (+) Open
The CAN-High wire of a low-speed comfort bus (e.g. ISO 11898-3 / fault-tolerant CAN) is open. Single-wire fallback may keep some functions alive but a fault is logged because the bus is no longer fault-tolerant.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
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
Common causes
Ordered by how often each turns out to be the cause.
- CAN-High wire brokenCommon
- Gateway module routing CAN messages incorrectlyLess likely
- CAN terminating resistor (120 Ω) drifted out of specLess likely
Parts involved
- Low Speed CAN Wiring
- Body Control Module
U0020 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.