U0001 U0xxx · Network (U) High severity
High Speed CAN Communication Bus
The high-speed CAN bus is off-line or badly degraded. Multiple modules lose contact at once — which is the clue that tells you it is the bus, not a module.
Severity guidance: High — stop driving and diagnose now; risk of further damage
Symptoms you will notice
- Many unrelated warning lamps
- Modules not responding to the scan tool
- No-start or limp mode
Common causes
Ordered from most to least likely — start at the top.
- 1 CAN-H or CAN-L short to earth, battery or each other
- 2 Open circuit in the bus wiring
- 3 A single module dragging the bus down
- 4 Missing or wrong terminating resistor
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Count how many modules are unreachable — more than one means bus, not module.
- ✓ Measure resistance across CAN-H and CAN-L with the battery disconnected: roughly 60 Ω means both terminators are present.
- ✓ Measure CAN-H at ~2.5–3.5 V and CAN-L at ~1.5–2.5 V with the ignition on.
- ✓ Disconnect modules one at a time to find the one that restores the bus.
- ✓ Scope the waveform — a flat line plus one active line tells you which side is shorted.
Repair options
- Repair the shorted or open bus wiring
- Replace the faulty module that is dragging the bus
- Restore the terminating resistor
U0001 — frequently asked questions
How do I tell a bus fault from a dead module? +
If two or more unrelated modules are unreachable, suspect the bus. A single missing module points at that module's supply or wiring.
Why 60 Ω and not 120 Ω? +
There are two 120 Ω terminating resistors in parallel. Reading 120 Ω means one is missing; 40 Ω suggests a third has been added.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.