U0003 – High Speed CAN (+) Open indicates that the positive side of the vehicle’s high-speed CAN communication circuit (CAN-H) has an open circuit. This disrupts the communication flow between modules such as the PCM, ABS, TCM, EPS, and airbag control system. When CAN-H is open, data either becomes corrupted or fails to transmit entirely, triggering warning lights, performance issues, or total system loss. Although the engine may still run, an open CAN-H line can lead to unpredictable and unsafe behavior. This guide explains what U0003 means, its causes, symptoms, diagnosis steps, and the best repair options.
What Does U0003 Mean?
U0003 is a generic network communication code stored when the PCM, BCM, ABS, or another control module detects that the CAN-H circuit is open. High-speed CAN networks use two wires — CAN-H and CAN-L — twisted together to create a stable communication link at 500 kbps. If the CAN-H wire becomes broken, disconnected, or highly resistive, the voltage and signal amplitude no longer match the expected values.
This leads to signal corruption or complete loss of communication. Because many critical vehicle systems depend on the high-speed CAN bus, even a small break in the CAN-H line can cause widespread faults and module dropout.
Quick Reference
- OBD-II Family: U-Code (Network Communication)
- Scope: Generic
- System: High-Speed CAN Bus
- Difficulty Level: Advanced
- Estimated Repair Cost: €80–€500+
- Last Updated: 20 November 2025
Real-World Example / Field Notes
A sedan came into the workshop with ABS, traction, and power steering lights illuminated. The scan tool could only communicate with a few modules. U0003 was stored in the PCM and BCM. After inspecting the harness behind the front wheel arch, a broken CAN-H wire was found caused by rubbing against the fender liner. Repairing the break restored full communication instantly.
In another case, a customer complained of intermittent no-start conditions. U0003 appeared along with several lost communication codes. The root cause was a corroded splice inside the driver-side sill panel where water had accumulated. Rebuilding the splice corrected the CAN-H line and resolved the no-start issue permanently.
Symptoms of U0003 – High Speed CAN (+) Open
- Multiple warning lights: ABS, traction control, airbag, or EPS lights activate.
- No communication with one or more modules: Scan tool cannot access certain controllers.
- Intermittent stalling: PCM loses critical data from other modules.
- No-start condition: Immobilizer or PCM data may be missing.
- Gauges behaving erratically: Cluster may flicker, freeze, or drop to zero.
- Transmission stuck in limp mode: TCM cannot receive engine information correctly.
- Loss of power steering: EPS module may drop offline.
Common Causes of U0003
Most Common Causes
- Open circuit in CAN-H wiring
- Corroded or loose connector pins on CAN-H line
- Broken or damaged harness from vibration or abrasion
- Water intrusion affecting wiring splices
- Rodent damage creating open wires
Less Common Causes
- Damaged termination resistor inside a module
- Incorrect aftermarket accessory wiring spliced into CAN lines
- Overheated or internally failing control module causing CAN-H dropout
- Poor chassis or module grounding affecting CAN-H voltage
- Connector back-out where pins slide out of position
- Battery voltage instability causing network sensitivity
Diagnosis: Step-by-Step Guide
To diagnose U0003 correctly, you must test wiring integrity, voltage levels, and individual module communication. Patience and a systematic approach are essential.
Tools You’ll Need
Advanced scan tool, multimeter, oscilloscope (recommended), wiring diagrams, CAN breakout box, backprobe kit, test light, and battery/alternator tester.
- Scan all modules: Note which ones communicate and which don’t. An open CAN-H line often isolates part of the network.
- Check battery voltage and alternator output: Low voltage or ripple can make CAN faults worse.
- Perform visual inspection: Look for damaged wiring, corrosion, or water intrusion along main CAN harness paths.
- Measure resistance between CAN-H and CAN-L: Should be ~60 ohms with key off. If reading is higher than normal, CAN-H may be open.
- Test CAN-H to ground resistance: Open or infinite resistance indicates a break in the circuit.
- Check CAN-H voltage: With ignition on, CAN-H should read ~3.5V. A reading near 2.5V or flatline suggests an open.
- Inspect connectors and splices: Pay attention to common failure points such as kick panels, footwells, and wheel wells.
- Perform wiggle test: Gently move harness sections while monitoring live CAN data for dropouts.
- Unplug modules one at a time: Helps isolate whether a module is dragging down the line.
- Use oscilloscope to inspect waveform: An open CAN-H line typically shows a distorted or absent differential signal.
Pro Tip: If the network shows 120 ohms instead of 60 ohms during resistance testing, one entire CAN line — usually CAN-H — is open between termination points.
Possible Fixes & Repair Costs
- Repair open CAN-H wire: Solder and heat-shrink repairs (€50–€200).
- Replace corroded connectors: Connector repair kits (€20–€100).
- Rebuild corroded splices: Common in footwell and sill areas (€50–€150).
- Fix water intrusion: Repair sunroof drains, seals, or grommets (€80–€250).
- Replace damaged harness sections: If wires are badly degraded (€150–€400).
- Replace faulty module: If internal failure affects CAN-H (€150–€600+).
Can I Still Drive With U0003?
Driving with U0003 may be risky depending on which modules lose communication. Loss of ABS, traction control, EPS, or transmission communication can make the vehicle unsafe. If you experience stalling, loss of steering assist, or multiple warning lights, avoid driving until the issue is repaired.
Related Codes
- U0073 – Control Module Communication Bus Off
- U0009 – High Speed CAN Communication Bus (-) Shorted to Bus (+)
- 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
- U0002 – High Speed CAN Communication Bus Performance
- U0001 – High Speed CAN Communication Bus
Key Takeaways
- U0003 indicates an open circuit on the high-speed CAN-H wire.
- Corrosion, broken wiring, or connector issues are the most common causes.
- Voltage and resistance checks help pinpoint the exact failure.
- Driving with communication faults can be unsafe depending on modules affected.
FAQ
How does U0003 differ from U0001?
U0001 indicates total CAN bus failure. U0003 specifically identifies an open circuit in the CAN-H line, which is a more precise wiring-related issue.
Can a bad battery trigger U0003?
Yes. Low voltage or alternator noise can worsen CAN performance and make wiring issues more noticeable.
Is U0003 dangerous to drive with?
It can be. If modules like ABS, EPS, or TCM lose communication, the vehicle may become unsafe.
Can I fix U0003 myself?
You can inspect wiring and connectors, but diagnosing CAN bus issues usually requires advanced tools and should be handled by a qualified technician.
Does U0003 always mean the wire is broken?
Usually, but it can also be caused by severe corrosion, loose pins, faulty splices, or a module that stops pulling the proper load on CAN-H.