OBD-II trouble code
U0009: High Speed CAN Communication Bus (-) Shorted to Bus (+)
The two high-speed CAN wires — CAN-Low and CAN-High — are shorted together. This collapses the bus voltage and disrupts communication for nearly every module on the network. Usually a wiring or connector fault.
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 U0009 mean?
U0009 is a specific high-speed CAN bus fault: the CAN-Low wire (often called CAN minus, or CAN-) is shorted to the CAN-High wire (CAN plus, or CAN+). High-speed CAN is the digital backbone of a modern vehicle — the engine computer, transmission controller, ABS module, instrument cluster, and many others share one twisted pair of wires, CAN-High and CAN-Low, to trade thousands of messages per second. The two wires are meant to carry mirror-image signals; the receiving modules read the difference between them. When the two wires touch each other, that difference disappears and the network can no longer tell a '1' from a '0'.
Normally CAN-High and CAN-Low both rest near 2.5 volts and then split apart during signaling — CAN-High rising toward about 3.5 volts and CAN-Low dropping toward about 1.5 volts. A short between the two wires forces them to the same voltage, so the bus can never reach the voltage split it needs to send data. Communication for every module on that bus degrades or stops, which is why a single short can make it look like the whole car has failed. The cause is almost always physical: a chafed harness where the insulation has worn through, crushed or pinched wiring, rodent damage, water and corrosion bridging two terminals in a connector, or a damaged module internally tying the two lines together.
Because high-speed CAN carries powertrain and safety data, U0009 is a serious fault. The vehicle may crank but not start, stall and refuse to restart, or drop multiple safety systems at once. A scan tool may also struggle to reach several modules. This is a code to diagnose methodically with a meter and wiring diagram rather than by replacing parts on a guess.
Common causes
- CAN-High and CAN-Low wires shorted together by chafed or worn insulation
- Crushed or pinched harness section pressing the two wires together
- Rodent damage to the CAN wiring
- Water intrusion and corrosion bridging two terminals in a connector
- Damaged splice pack where multiple CAN branches join
- A failed module internally shorting CAN-High to CAN-Low
- Pierced or improperly tapped wiring from a prior accessory installation
- Connector terminals bent or pushed together during a repair
Symptoms
- Multiple warning lights illuminated at once (check engine, ABS, traction, airbag)
- Engine cranks but will not start, or stalls and will not restart
- Gauges dead, frozen, or erratic
- Transmission stuck in limp mode or refusing to shift
- Scan tool cannot communicate with several modules
- Several module-specific U-codes stored alongside U0009
- Problem may come and go with road vibration if the short is intermittent
Diagnostic steps
- 1.Record every stored code in all modules — the pattern of which modules report faults helps localize the shorted segment.
- 2.With the key off and battery disconnected, measure resistance across CAN-High and CAN-Low at the OBD port. A healthy two-terminator bus reads about 60 ohms; a reading near 0 ohms confirms the two wires are shorted together.
- 3.Inspect the CAN harness for chafing, crush damage, rodent damage, and pinch points, paying attention to areas where the harness passes brackets or moves with the body.
- 4.Open suspect connectors and check for water intrusion, corrosion, and terminals that have been pushed together.
- 5.Disconnect modules one at a time while watching the bus resistance or communication; if the short clears when a specific module is unplugged, that module or its branch wiring is the fault.
- 6.Use an oscilloscope on CAN-High and CAN-Low to confirm the lines are collapsed together rather than splitting during signaling.
Repair cost
$100 – $1,200
Diagnosis alone often runs $150-$300 because isolating a bus short is methodical work. Repairing a chafed or shorted section of wiring or a corroded connector typically lands at $150-$600 depending on access. Replacing a module that is internally shorting the bus, including programming, runs $400-$1,000. European and luxury vehicles with multiple bus segments cost more to diagnose.
Estimate your repair
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Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
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.