U0008 – High Speed CAN Communication Bus (-) High

Last updated: April 7, 2026

U0008 means the vehicle’s high-speed CAN network has a “bus minus” line that the module sees as too high, which can make modules drop offline and cause multiple warning lights. You may notice no-start, stalled engine, or a dash full of alerts. According to factory diagnostic data, this code indicates “High Speed CAN Communication Bus (-) High.” The description sounds vague on purpose. SAE J2012-DA U-codes stay general so many manufacturers can reuse them. Your job in diagnosis is to prove which module or wiring segment drives the CAN (-) line high before you replace anything.

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U0008 Quick Answer

U0008 points to an abnormal high voltage condition on the high-speed CAN Bus (-) circuit. Start with a full network scan, then verify power/ground integrity and CAN wiring for shorts to voltage.

What Does U0008 Mean?

U0008 code means a control module detected a problem on the high-speed CAN communication network, specifically the CAN (-) side reading “high.” In real terms, the vehicle can lose communication between modules. That can disable systems like ABS, traction control, electronic steering assist, or transmission functions. The exact symptoms depend on which modules miss messages.

Officially, the definition is “High Speed CAN Communication Bus (-) High.” The module does not “see” a bad part. It monitors the electrical state of the CAN (-) circuit while it expects normal network signaling. When CAN (-) stays higher than expected, messages corrupt or stop. Diagnosis must identify whether a harness fault, a connector issue, or a module pulling the line high caused the condition.

Theory of Operation

The high-speed CAN bus uses two wires that carry opposing signals: CAN (+) and CAN (-). Every module shares those wires and broadcasts messages at set intervals. Each module also has internal circuitry that biases the bus and shapes the signal. Termination and proper wiring keep the signal stable and prevent reflections.

U0008 sets when a module decides the CAN (-) line remains too high for reliable communication. A short to battery voltage, water intrusion in a connector, or chafed wiring can drive CAN (-) high. A single module with an internal fault can also pull the network off its normal bias. Because all modules share the same pair, one bad point can affect the entire vehicle.

Symptoms

U0008 symptoms usually show up as network-wide glitches, not one isolated sensor failure.

• Scan tool issues: One or more modules do not respond, disappear from the module list, or drop in and out during a network scan.
• Multiple warning lights: ABS, traction control, airbag, power steering, and MIL may illuminate together.
• No-start or stall: The engine may crank and not start, or stall after starting if key modules stop sharing data.
• Transmission failsafe: The vehicle may stay in one gear, shift harshly, or show a PRNDL mismatch.
• Inoperative gauges: Speedometer, tachometer, or warning messages may flicker or go blank.
• Intermittent accessories: HVAC controls, windows, or infotainment can reset when the network drops.
• Loss of driver aids: Cruise control, stability control, or lane/parking features may disable due to missing messages.

Common Causes

• CAN (-) short to battery voltage: Battery feed contacting CAN (-) drives the line high and corrupts High Speed CAN traffic.
• CAN (-) short to CAN (+): A chafed harness can tie the pair together and force both lines toward the same voltage, which breaks differential signaling.
• Open/high resistance in the CAN (-) conductor: A partially broken wire or spread terminal can let the network bias pull CAN (-) high and trigger the “(-) high” condition.
• Aftermarket device loading the High Speed CAN: Remote starts, trackers, and audio interfaces can backfeed or clamp CAN (-), especially with poor splices.
• Water intrusion or corrosion in a backbone connector: Moisture creates unintended conductivity between CAN (-) and power or other circuits, which drives CAN (-) high.
• Module internal transceiver fault: A failing CAN transceiver inside any module on the HS-CAN can hold CAN (-) high and take the whole bus down.
• Missing or compromised termination on the HS-CAN: An unplugged module at a terminating location or damaged termination wiring can distort bus bias and cause invalid voltages.
• Poor power or ground to a critical network module: Low module supply or a high-resistance ground can make a transceiver output abnormal levels and upset CAN (-).

Diagnosis Steps

You need a scan tool with a full network scan, not just generic OBD-II. Use a DVOM for voltage-drop and bias checks. Have access to wiring diagrams and connector views for the HS-CAN pair. An oscilloscope helps confirm bus waveform quality, but you can diagnose most U0008 faults with resistance and bias voltage tests.

1. Confirm U0008 and record all DTCs from every module. Save freeze frame data and note ignition state, vehicle speed, and any companion U-codes. Freeze frame shows the conditions when U0008 set; a scan tool snapshot can capture an intermittent dropout during a road test.
2. Run a full network scan and document which modules do not report. If the scan tool cannot communicate with multiple modules, treat this as a bus-level problem first. If only one module drops out, focus on that module’s connector, power, ground, and local splice points.
3. Check for pending versus confirmed/stored U0008. A pending code suggests an intermittent event, often harness movement or moisture. A confirmed code that returns immediately on key-on points to a hard short, open, or a module holding the line high.
4. Inspect fuses and power distribution that feed network-critical modules before probing any ECU connectors. Verify fuse integrity under load when possible, not just visually. Pay special attention to ignition-fed circuits that wake modules and enable CAN bias.
5. Voltage-drop test module powers and grounds under load on the most suspicious or most “central” module you can access. Load the circuit by commanding actuators, turning on headlights, or using scan tool output controls. Keep ground voltage drop below 0.1V while the circuit operates; high resistance can pass a continuity test and still crash the network.
6. Perform a targeted visual inspection of the HS-CAN twisted pair and its connectors. Focus on areas with movement or repairs: under-battery trays, radiator support, firewall pass-throughs, under- harnessing, and collision repair zones. Look for crushed conduit, shiny rub marks, green corrosion, and non-OEM splices.
7. Key OFF, disconnect the battery, and let modules go to sleep. Measure resistance between CAN (+) and CAN (-) at an accessible HS-CAN connector. A healthy network reads about 60 ohms; readings near 120 ohms, OL, or very low resistance point to an open, missing termination, or a short.
8. With the battery reconnected and ignition ON, measure CAN (+) and CAN (-) bias voltage to ground at the same location. Use ignition ON because bias voltage does not exist with the network asleep. A healthy HS-CAN typically sits near 2.5V on both lines; a significantly higher CAN (-) than expected supports the U0008 “(-) high” fault direction.
9. Isolate the fault by unplugging modules one at a time while watching CAN resistance (key OFF, battery disconnected) and then bias voltage (ignition ON). Start with recently serviced areas and easy-access modules. If unplugging one module restores about 60 ohms and normal bias, that module or its branch wiring likely holds CAN (-) high.
10. If the bus checks good at the DLC but fails at another module connector, locate the harness break between those points. Use splice pack locations and connector pin fits to narrow the segment. Perform pin-tension checks on CAN terminals; a loose terminal can act like an open and let the line float high.
11. After repairs, clear DTCs and rerun a full network scan. Confirm all modules communicate and U0008 stays out after a key cycle and a road test. If the concern was intermittent, use a scan tool snapshot during the drive to capture the moment communication degrades.

Professional tip: When U0008 shows “CAN (-) high,” don’t assume the CAN (-) wire is shorted. A single open or high-resistance terminal on CAN (-) can let the network bias pull it upward. Always confirm with the 60-ohm check and then isolate branches by unplugging modules.

Possible Fixes

• Repair chafed wiring or a short to B+ affecting the High Speed CAN (-) circuit, then rewrap and secure the harness away from sharp edges.
• Clean and repair corrosion or water intrusion in a backbone connector or splice pack, then apply proper terminal service procedures and sealing.
• Restore proper terminal fit by replacing spread CAN terminals and correcting pin tension at the affected connector.
• Remove or rewire an aftermarket device that backfeeds or loads the HS-CAN, using correct connectors instead of scotch-lock style taps.
• Repair power or ground faults to a module that destabilize the CAN transceiver, verified with voltage-drop testing under load.
• Replace a module only after isolation proves it holds CAN (-) high and wiring, power, ground, and terminations test good.

Can I Still Drive With U0008?

You can often drive with a U0008 code, but you should treat it as a reliability risk. U0008 means a module detected the high-speed CAN bus negative line reading higher than expected, so messages can corrupt or drop out. Some vehicles only lose comfort features, like radio or HVAC controls. Others can lose critical functions if a powertrain, ABS, or steering module drops off the network. If the vehicle stalls, shifts harshly, loses the speedometer, or shows multiple warning lamps, stop driving and diagnose it. Intermittent U0008 faults can change behavior from one key cycle to the next.

How Serious Is This Code?

U0008 ranges from an inconvenience to a safety concern, based on which modules share that high-speed bus. When the fault only affects non-critical modules, you may only see warning messages and feature dropouts. Severity rises fast when ABS, ESC, EPS, or the PCM share the same bus, because lost data can disable stability control or change shift strategy. A “bus (-) high” condition also points toward wiring or a module fault that can worsen with heat, vibration, or moisture. Address U0008 promptly if you see drivability changes, repeated no-starts, or many U-codes setting together.

Common Misdiagnoses

Technicians often replace a “non-communicating” module too early. A bus (-) high fault can make a good module appear dead. Another common mistake involves chasing the first U-code listed, while ignoring that U0008 describes the network’s electrical state. Shops also miss power and ground voltage-drop testing under load for key modules, which can push the transceiver out of bias and mimic a bus fault. Corrosion in splice packs and connectors causes intermittent highs that disappear during a quick bay check. To avoid wasted spending, confirm network health, isolate the segment, then prove the fault follows a harness, connector, or module.

Most Likely Fix

The most common confirmed repair directions for U0008 involve wiring and connection integrity, not immediate module replacement. Many fixes start with locating a high-speed CAN harness section with water intrusion, crushed insulation, or corrosion at a splice, then restoring proper terminals and sealing. Another frequent direction involves repairing a weak module power or ground that distorts bus bias, especially when loads turn on. If isolation testing shows the bus returns to normal only when a specific module is unplugged, that module or its connector pins become the next test focus, not an automatic replacement.

Repair Costs

Network and communication fault repairs vary by root cause — wiring/connectors are often the source, but module-level repairs or replacements can be significantly more expensive.