OBD-II trouble code
U0003: High Speed CAN Communication Bus (+) Open
A break in the CAN-High conductor. An open does not kill the network the way a short does — it cuts it in half, and each half carries on talking to itself while reporting that the other half has vanished.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $800
- DIY difficulty
- Shop recommended
What does U0003 mean?
A high-speed CAN network is a line, not a star. The two conductors run from one end of the vehicle to the other with modules tapped off along the way, and there is a 120-ohm terminating resistor at each end — usually built into two of the modules rather than existing as a separate part. Measured across the pair, those two resistors sit in parallel and give the 60 ohms that every diagnostic guide tells you to look for.
U0003 stores when the CAN-High conductor is broken. What follows from that is topological rather than electrical, and it is the single most useful thing to understand about this code: the bus does not go silent. It becomes two buses. Everything on one side of the break can still reach everything else on that side, because they still share both conductors between themselves and one of the two terminators is still with them. Everything on the far side of the break does the same. Neither group can reach the other, so every module on each side starts logging lost-communication codes naming the modules on the other side.
That produces a distinctive picture at the scan tool, and it is worth recognising because it points straight at the location of the break. The tool talks through the diagnostic connector, so it can only see the segment the connector happens to be attached to. You will find one coherent group of modules that respond normally and appear healthy, and another group that are simply not there — and the modules that are present will be complaining about exactly the ones that are missing. Draw the two lists against the wiring diagram and the break is between them. That is far more directive than the usual advice to inspect the harness.
The measurement that confirms it is arithmetic. With the ignition off, measure across CAN-High and CAN-Low at the diagnostic connector. A healthy bus reads close to 60 ohms. A CAN-High break between the connector and one of the terminators leaves you measuring only the terminator on your side, so the reading is close to 120 ohms. That single number distinguishes an open from a short before any trim is removed, and it also tells you which side of the vehicle to start on, because the segment you can measure is the segment the connector is on.
One caution about expectations. Some vehicles run a low-speed, fault-tolerant CAN network alongside the high-speed one, and fault-tolerant transceivers are specifically designed to keep working on a single wire when one conductor fails. That is a real capability and it is not available here. High-speed CAN has no single-wire fallback, so an open on this bus is not a degraded mode — it is a split network, and the segments stay split until the conductor is repaired.
Breaks concentrate where the harness moves or has been worked on: door and tailgate boots, engine harness connectors that flex on the mounts, splice packs, and any previous repair. Terminals that have spread or backed out of a connector body behave exactly like a cut wire and are much easier to miss.
Common causes
- Broken CAN-High conductor inside a door, tailgate, or engine harness boot that flexes in service
- Terminal backed out or spread inside a connector body, which behaves identically to a cut wire
- Corroded or failed splice pack, where several branches of the bus join
- Poor previous repair — a crimp that has pulled apart or a splice that has corroded internally
- Connector left unlatched after body, dash, or drivetrain work
- Harness cut or pierced during accessory fitment or bodywork
- Rodent damage, which is disproportionately common on harnesses routed near the bulkhead
- Water ingress into an inline connector, corroding one pin of the pair
- Module with the terminating resistor removed from the network, which changes the reading without a broken wire
Symptoms
- Multiple lost-communication codes, with each surviving module naming the ones on the far side of the break
- Scan tool that connects and reports a coherent group of modules while others are entirely absent
- Several warning lamps on at once — ABS, traction, airbag, and powertrain together
- Instrument cluster gauges dead or stuck while the engine still runs
- No-start or crank-no-start where the segment containing the immobiliser is cut off
- Transmission stuck in a default gear or refusing to shift
- Fault that appears after a door, tailgate, or bodywork repair
- Symptoms that change as a door is opened and closed, if the break is in a door boot
Diagnostic steps
- 1.Note which modules the scan tool can see and which it cannot, then list which modules each responding one reports as missing. The break lies between the two groups on the wiring diagram.
- 2.Measure resistance across CAN-High and CAN-Low at the diagnostic connector with the ignition off. Roughly 60 ohms is normal; roughly 120 ohms indicates the bus is split and only one terminator is on your side.
- 3.Use the 120-ohm reading directionally — the segment you can measure is the one containing the diagnostic connector, so search away from it toward the missing modules.
- 4.Do not expect a single-wire fallback. High-speed CAN has none, so a working subset of modules is evidence of a split network rather than of a partially working one.
- 5.Inspect harness boots at the doors, tailgate, and engine mounts first, flexing them by hand while watching a meter or the module list.
- 6.Open connectors along the suspect segment and check for terminals that have retracted or spread, which look perfectly normal from outside the housing.
- 7.Inspect any splice pack on the bus. A corroded splice reproduces this fault exactly and is not visible without unwrapping.
- 8.Check anything recently repaired or fitted, including aftermarket accessories, since a pierced or cut conductor is a common cause and the timing usually gives it away.
- 9.Confirm both terminating resistors are still present by measuring each segment separately once the break is located, so a missing terminator is not mistaken for a second fault.
- 10.Repair with a short, properly crimped and sealed joint, and keep the pair twisted through the repair.
Repair cost
$100 – $800
Diagnosis runs $150 to $300, and the module-list comparison described above is what keeps it at the lower end — knowing which two modules the break sits between converts a whole-vehicle search into a specific one. A conductor or terminal repair is $150 to $500 depending on access, with a door or tailgate boot at the cheaper end and a section buried behind the dash or under the floor at the upper. Splice pack repair falls in the same band. There is usually no part to buy on this code, which is why diagnostic accuracy is where the money is either saved or wasted.
Estimate your repair
Run the numbers for your vehicle
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.