High Speed CAN Communication Bus Negative High
U0008 means High Speed CAN Communication Bus Negative High. Review likely symptoms, causes, safety considerations, and a sensible diagnostic order.
What you may notice
Symptoms
- 01Several warning lights or unrelated codes appearing together
- 02A module is missing from the scan-tool network list
- 03Intermittent no-start, no-shift, steering, braking, or comfort-system operation
- 04Functions may return after cycling the ignition
Where to look first
Common causes
- 01Low battery voltage, unstable charging voltage, or poor module ground
- 02Open, shorted, corroded, or damaged CAN bus wiring
- 03Blown fuse, failed relay, or missing power/ground at the named module
- 04A module holding the network down or failing internally
A sensible sequence
Best diagnostic order
- 01Save the full-vehicle scan and note current versus history codes
- 02Stabilize battery voltage and check fuses, powers, grounds, and recent repairs
- 03Identify the network and the module that cannot be reached
- 04Inspect the shared bus and isolate branches only as directed by service information
- 05Restore communication, clear secondary codes, and verify all modules remain online
Step by step
What to check and repair
- 01Perform a complete module scan before clearing any codes
- 02Verify battery condition and module powers and grounds
- 03Compare which modules report the communication fault and which module is absent
- 04Test bus voltage, resistance, topology, and connectors using the vehicle diagram
Read the signals
Live-data clues
Module network list
- Typical behavior
- Every module fitted to the vehicle should respond consistently
- Possible clue
- One absent module suggests its power, ground, branch wiring, connector, or internal electronics
CAN bus voltage and waveform
- Typical behavior
- Both lines should show a clean complementary signal within vehicle specifications
- Possible clue
- A line fixed high, low, open, or distorted points toward wiring, termination, or a module loading the bus
| Parameter | Typical behavior | What it can suggest |
|---|---|---|
| Module network list | Every module fitted to the vehicle should respond consistently | One absent module suggests its power, ground, branch wiring, connector, or internal electronics |
| CAN bus voltage and waveform | Both lines should show a clean complementary signal within vehicle specifications | A line fixed high, low, open, or distorted points toward wiring, termination, or a module loading the bus |
What U0008 means
U0008 is a diagnostic trouble code in the U family. Its standardized description is “High Speed CAN Communication Bus Negative High.” The code identifies the monitored circuit, module, or condition; it does not prove that one named part must be replaced.
Diagnostic direction
A U-code usually records a communication symptom, not automatic proof that the named module has failed. Battery voltage, powers, grounds, shared wiring, gateway operation, and which module set the code determine the diagnostic path.
Before clearing the code
Save the complete scan, including every module that reported the fault and any failure-type or symptom byte. Check vehicle-specific service information because module names, network topology, connector locations, and enable conditions vary by vehicle. Clear the code only after the verified cause is repaired, then repeat the relevant self-test or drive cycle.