U0009 – CAN (–) Shorted to CAN (+) indicates a direct electrical short between the two wires of the high-speed CAN network: CAN-H (positive) and CAN-L (negative). Because these wires must carry opposite signals to create a stable differential data stream, any short between them instantly disrupts communication across the entire network. This can cause stalling, loss of ABS or steering assist, multiple warning lights, or a complete no-start condition. This guide explains what U0009 means, its symptoms, causes, diagnosis steps, and the most effective repair options.
What Does U0009 Mean?
U0009 is a generic OBD-II communication code stored when the vehicle’s Powertrain Control Module (PCM) or another controller detects that the CAN-H and CAN-L wires are making electrical contact. In normal operation, the two lines carry opposite voltage signals (CAN-H ~3.5V and CAN-L ~1.5V). When these lines short together, the differential signal collapses, making communication impossible.
This type of fault usually results from wiring damage, crushed harnesses, water intrusion, or internal module short-circuits. Because nearly every module relies on CAN communication, U0009 can disable major safety and drivability systems instantly.
Quick Reference
- OBD-II Family: U-Code (Network Communication)
- Scope: Generic
- System: High-Speed CAN Bus
- Difficulty Level: Advanced
- Estimated Repair Cost: €100–€600+
- Last Updated: 20 November 2025
Real-World Example / Field Notes
A vehicle arrived on a tow truck after stalling abruptly in traffic. Almost no communication with modules was possible. The PCM stored U0009 along with several “No Communication” faults. Inspecting the main engine harness revealed that the CAN wiring had melted against a hot exhaust component, causing CAN-H and CAN-L to fuse together. Repairing the wiring and insulating the loom restored full function.
Another case involved water flooding the rear trunk compartment of a luxury sedan. A junction connector submerged in water caused severe corrosion, bridging CAN-H and CAN-L internally. U0009 appeared across all major modules. Replacing the connector and sealing the water entry point resolved the issue permanently.
Symptoms of U0009 – CAN (–) Shorted to CAN (+)
- Multiple warning lights: ABS, traction, airbag, EPS, and check engine may all activate.
- No communication with modules: Scan tool cannot establish a stable connection.
- Intermittent or permanent no-start: PCM and immobilizer cannot exchange data.
- Stalling while driving: Critical modules drop offline.
- Loss of power steering assist: EPS shuts down without CAN data.
- Transmission limp mode: TCM loses engine load information.
- Cluster outages: Gauges freeze or drop to zero.
Common Causes of U0009
Most Common Causes
- CAN-H and CAN-L wires physically touching each other
- Crushed or pinched wiring harnesses
- Melted wiring from heat or exhaust contact
- Water intrusion bridging CAN wires at connectors
- Rodent damage exposing and crossing wires
Less Common Causes
- Faulty module shorting CAN-H to CAN-L internally
- Corroded splices causing cross-connection
- Incorrect aftermarket wiring tied into CAN lines
- Electrical overload or short from another circuit bleeding into CAN wiring
- Failure of a terminating resistor inside a module
Diagnosis: Step-by-Step Guide
Because CAN-H and CAN-L share a direct short in U0009, the fault often affects the entire network. Systematic testing is essential to isolate the exact location of the short.
Tools You’ll Need
Advanced scan tool, multimeter, oscilloscope, wiring diagrams, CAN breakout box, backprobe kit, test light, and battery/alternator tester.
- Scan all modules: Identify how many modules are offline. A total blackout suggests a major short.
- Disconnect battery: Prevent further module damage during testing.
- Measure resistance between CAN-H and CAN-L: With key off, resistance will be near 0 ohms if the lines are shorted.
- Visually inspect wiring: Focus on engine bay, wheel wells, under seats, and trunk compartments.
- Inspect connectors: Look for corrosion bridging pins, especially in wet areas.
- Unplug modules one at a time: If the short disappears after a module is disconnected, the module may be internally shorted.
- Trace wiring continuity: Check segments between modules to find crushed or melted sections.
- Check for aftermarket wiring: Alarms, audio systems, and trackers often cause CAN shorts.
- Use oscilloscope: Before repair, waveform will be flat or identical on both lines.
- Repair short: Once found, repair wiring and clear codes. Re-test communication.
Pro Tip: If CAN-H and CAN-L both show ~2.5V at all times, it usually indicates a direct short between the two conductors.
Possible Fixes & Repair Costs
- Repair wiring where CAN-H and CAN-L touch: €60–€200
- Replace corroded connectors: €20–€100
- Replace damaged harness sections: €150–€400
- Repair water leaks causing intrusion: €80–€250
- Replace faulty module shorting CAN internally: €150–€600+
- Remove incorrect aftermarket wiring: €50–€150
Can I Still Drive With U0009?
No. U0009 is one of the most serious CAN network faults. A direct short between CAN-H and CAN-L can disable ABS, power steering, airbags, engine control, and immobilizer functions. Driving may be unsafe or impossible. The vehicle should not be driven until the fault is repaired.
Related Codes
- U0073 – Control Module Communication Bus Off
- U0008 – High Speed CAN Communication Bus (-) High
- U0007 – High Speed CAN Communication Bus (-) Low
- U0006 – High Speed CAN Communication Bus (-) Open
- U0005 – High Speed CAN Communication Bus (+) High
- U0004 – High Speed CAN Communication Bus (+) Low
- U0003 – High Speed CAN Communication Bus (+) Open
- U0002 – High Speed CAN Communication Bus Performance
- U0001 – High Speed CAN Communication Bus
Key Takeaways
- U0009 indicates a direct short between CAN-H and CAN-L.
- This is a severe CAN-bus failure that disables communication network-wide.
- Most causes are wiring-related: crushed, melted, corroded, or flooded harnesses.
- Driving is unsafe — repair before use.
FAQ
Is U0009 the most serious CAN code?
Yes. A direct short between CAN-H and CAN-L can collapse the entire network, causing multiple critical systems to fail simultaneously.
Does U0009 always mean wiring damage?
Usually, but an internal module short can also bridge the lines. Always isolate by unplugging modules one at a time.
Can I clear U0009 and keep driving?
No. The short remains physically present and can lead to stalling or loss of steering/braking communication.
Can water intrusion cause U0009?
Yes. Corrosion in connectors or junction blocks is a very common cause of shorts between CAN wires.
Do I need an oscilloscope to diagnose U0009?
It helps, but a multimeter and resistance checks are often enough to identify the short.