U0021 means the low-speed Controller Area Network (CAN) bus has an open circuit somewhere in its wiring or connections, preventing one or more control modules from communicating reliably. Low-speed CAN usually handles body and convenience systems, so this code points to a network integrity problem rather than a single sensor failure.
A break, chafe, pinched wire, corroded splice, or poor pin connection in the CAN-H or CAN-L lines is the most common reason the bus reads as open.
A module connected to the low-speed network can fail internally and pull the bus down or create an open, preventing other modules from communicating.
Water intrusion, terminal spread, or oxidation at a module connector or splice pack can interrupt the CAN signal and set this code.
If the low-speed bus relies on specific termination and it is disconnected or damaged, the network may report an open bus condition.
A module that loses proper power or ground may appear to create a communication open, even though the CAN wires are intact.
This is uncommon; the engine computer is usually not the cause of a low-speed CAN open, and should only be suspected after wiring and all other modules are ruled out.
Usually yes if the engine runs normally, but body and convenience functions may be unreliable. It should be diagnosed to avoid losing features you rely on.
Almost never. U0021 is a network wiring or module communication issue, and replacing the engine computer is usually the wrong repair.
When the CAN bus is open, several modules may set communication or lost-communication codes. Many of those secondary codes often clear once the bus is repaired.
It usually does not stop the engine or transmission from working, but it can disable safety or convenience features. The severity depends on which systems are on the affected bus.