U0021: The low speed CAN network (high line) has a broken or disconnected wire
The low speed CAN network (high line) has a broken or disconnected wire. It is usually a broken wire, a loose connector or the part itself.
| Code | U0021 |
|---|---|
| System | Communication network |
| Group | U00xx, Communication buses |
| Type | Generic: it means the same on every make |
How to read and clear U0021 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see U0021 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- U0017 The medium speed CAN network (low line) gives a higher signal than normal
- U0018 The negative line of the medium speed CAN bus is shorted to the positive line
- U0019 The low speed CAN network is not working properly
- U0020 The low speed CAN network gives readings outside the normal range
- U0022 The low speed CAN network (high line) gives a lower signal than normal
- U0023 The low speed CAN network (high line) gives a higher signal than normal
- U0024 The low speed CAN network (low line) has a broken or disconnected wire
- U0025 The low speed CAN network (low line) gives a lower signal than normal
- U0001 High speed CAN communication bus
- U0073 Control module communication bus A off
What does code U0021 mean?
The low speed CAN network (high line) has a broken or disconnected wire. It is usually a broken wire, a loose connector or the part itself.
How do you clear U0021?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, U0021 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
