OBD-II trouble code
U0008: High Speed CAN Communication Bus (-) High
The CAN-Low (negative) line of the high-speed bus is reading too high — usually shorted to a power source or to CAN-High. The elevated voltage corrupts the differential and disrupts the network.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $1,200
- DIY difficulty
- Shop recommended
What does U0008 mean?
U0008 means the high-speed CAN bus's negative line — CAN-Low or CAN(-) — is sitting at a voltage that's too high for valid communication. CAN-Low should rest near 2.5 volts and dip toward roughly 1.5 volts only briefly when a module signals. If the line is instead pulled up — for example shorted to battery voltage or to the CAN-High wire — the differential between the two conductors collapses and modules can no longer decode bus messages.
This is the 'high' counterpart to U0007 (CAN-Low too low) and U0006 (CAN-Low open). A high condition usually traces to a short to a power source or a short between CAN-Low and CAN-High: a chafed wire touching a hot conductor, water bridging a connector to a powered pin, or harness damage. Occasionally a failed module drives the line high. The modules that detect the corrupted bus store U0008, frequently with module-specific lost-communication codes alongside.
Because CAN connects the engine, transmission, ABS, instrument cluster, and body modules, a high CAN-Low line can disable several systems simultaneously. Expect multiple warning lights, possible no-start or limp mode, and a scan tool that can't reliably reach the affected modules. A short to battery voltage can also damage modules on the bus, so the repair sometimes extends beyond the wiring. U0008 is diagnosed with a meter and wiring diagram, not by guessing at parts.
Common causes
- CAN-Low wire shorted to battery voltage or a switched power circuit
- CAN-Low shorted to CAN-High, collapsing the differential
- Chafed CAN-Low conductor contacting a hot wire in the harness
- Water intrusion bridging CAN-Low to a powered terminal in a connector
- Harness damage from an accident or repair bringing the bus into contact with power
- A failed module driving the CAN-Low line high
- Incorrectly repaired or spliced harness routing power onto the bus
Symptoms
- Multiple warning lights illuminated at once (check engine, ABS, traction, airbag)
- Engine may crank but not start, or run in limp mode
- Dead, frozen, or erratic gauges
- Scan tool cannot communicate with several modules
- Module-specific lost-communication U-codes stored with U0008
- In severe cases, more than one module damaged by the short to power
Diagnostic steps
- 1.Record all stored codes across every module to identify the affected segment.
- 2.Key on, measure CAN-Low voltage. It should rest near 2.5 volts; a reading pinned well above that points to a short to power or to CAN-High.
- 3.Trace the CAN-Low wire looking for chafe points, pinch damage, and contact with powered wires or terminals.
- 4.Inspect connectors for water intrusion and corrosion that could bridge CAN-Low to a hot pin.
- 5.With key off and battery disconnected, check bus resistance (about 60 ohms healthy) and test for unexpected continuity between CAN-Low and power or CAN-High.
- 6.Disconnect modules one at a time while watching CAN-Low voltage; if it normalizes when one is unplugged, that module may be driving the line high.
- 7.After clearing the short, scan all modules — a short to power can damage modules, which may need further repair.
Repair cost
$100 – $1,200
Diagnosis typically runs $150-$300. Clearing a short to power or between bus wires is often $150-$600. Because a short to battery voltage can damage modules on the bus, repairs sometimes extend to module replacement and programming at $400-$1,000+, higher on European and luxury vehicles.
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DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.