OBD-II trouble code
U0003: High Speed CAN Communication Bus (+) Open
The CAN-High (positive) wire of the high-speed communication bus is open — broken, disconnected, or interrupted somewhere in the harness. Modules lose the ability to talk to one another over that line.
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?
U0003 is a circuit-specific network code that means the high-speed CAN bus's positive line — usually labeled CAN-High or CAN(+) — has gone open. An 'open' is a break in the circuit: a cut or corroded wire, a backed-out terminal, a separated splice, or an unplugged connector. Where U0001 says 'the bus is sick somewhere,' U0003 narrows it to a specific electrical fault on the CAN-High conductor.
High-speed CAN uses two twisted wires, CAN-High and CAN-Low, that swing in opposite directions to carry each data bit as a voltage difference between them. When CAN-High is open, the modules downstream of the break can no longer see the high side of that differential signal, so messages stop getting through on the affected segment. The control modules that detect the loss store U0003, and you'll often see a cluster of module-specific 'lost communication' codes set at the same time because those modules genuinely fell off the network.
Because the bus is the backbone that ties the engine computer, transmission, ABS, instrument cluster, and body modules together, an open CAN-High line can take down several systems at once. The car may crank but not start, drop into limp mode, or refuse to communicate with a scan tool on the affected modules. U0003 is an electrical fault to be traced with a meter and a wiring diagram, not guessed at by swapping parts.
Common causes
- Broken or cut CAN-High wire from rodent damage, an accident, or chafing against a bracket
- Backed-out, spread, or corroded terminal at a module connector
- Separated or cold splice in the CAN-High conductor (splice packs are a common spot)
- Unplugged or partially seated connector after recent service
- Water intrusion and corrosion opening the circuit intermittently
- Open caused by a failed terminating resistor branch on the CAN-High side
- Harness damage near a connector boot where the wire flexes
Symptoms
- Multiple warning lights on at once (check engine, ABS, traction, airbag)
- Engine may crank but not start, or run in limp mode
- Gauges dead, frozen, or erratic
- Scan tool cannot reach one or more modules on the affected segment
- Several module-specific 'lost communication' U-codes stored together
- Symptoms that come and go with road vibration if the open is intermittent
Diagnostic steps
- 1.Record every stored code in all modules; the pattern of which modules dropped off helps locate the break in the CAN-High line.
- 2.With 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; an open on one branch often reads near 120 ohms or infinite.
- 3.Perform continuity checks on the CAN-High wire between connectors, working segment by segment to find the break.
- 4.Inspect connectors for backed-out, spread, or corroded terminals and reseat or repair as needed.
- 5.Wiggle-test the harness and connectors while watching bus communication to catch an intermittent open.
- 6.Use a wiring diagram to confirm which modules sit downstream of the suspected break and verify they are the ones that lost communication.
Repair cost
$100 – $800
Diagnosis often runs $150-$300 because tracing an open along the harness is methodical work. A wire or terminal repair is frequently $150-$500 depending on access. A splice-pack or connector repair lands in the same range; a buried harness section behind the dash or under the floor pushes labor toward the high end.
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.