OBD-II trouble code
U0006: High Speed CAN Communication Bus (-) Open
A break in the CAN-Low conductor. The reason CAN uses two twisted wires is noise cancellation, and losing one destroys it — so this fault often behaves erratically under electrical load rather than failing cleanly.
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 U0006 mean?
CAN is a differential protocol. Receivers do not care what voltage either wire sits at; they care only about the difference between them. That choice is what lets a data network survive inside a vehicle, because a car is an electrically filthy place — ignition coils, injectors, an alternator switching tens of amps, wiper and blower and fuel pump motors all radiating interference into every loom. Noise induced into a tightly twisted pair couples almost equally into both conductors, so it moves both by the same amount and vanishes when the receiver subtracts one from the other. That cancellation is not incidental; it is the whole reason for the second wire.
U0006 stores when the CAN-Low conductor is broken. Once it is, that cancellation is gone. What remains is a single wire running the length of the vehicle, which is a very effective antenna, and the receivers have nothing to subtract from it. This is why an open on this line frequently does not present as a clean, permanent failure. Instead it presents as a network that behaves acceptably in a quiet garage and falls apart when the vehicle is doing electrical work — under load with the alternator at high output, with the blower and headlights on, on a rough road that flexes the loom, or on a vehicle with a failing plug lead or a noisy motor nearby. Technicians who verify a repair at idle in a workshop and hand the car back to the same complaint have usually met exactly this.
A second consequence follows for anyone who repairs the fault. The twist rate of the pair is a design parameter, not packaging tidiness. Two conductors twisted at a consistent pitch are exposed to the same interference at every point along their length, which is what makes the cancellation work. Splice in a foot of untwisted wire and you have created a stretch of the bus that has no noise immunity at all, and you may hand the car back with a fault that behaves differently rather than one that is gone. Repairs on this bus should be short, crimped and sealed rather than soldered and taped, twisted through the joint, and routed exactly where the original ran — not tucked alongside a power feed, an injector harness, or an ignition lead because it was convenient.
Terminal plating matters here for the same reason connector integrity does elsewhere. CAN connectors commonly use gold-plated terminals precisely because contact resistance has to stay stable and low. Repairing one with a tin-plated terminal from a generic kit puts two dissimilar metals in contact in a damp environment, which corrodes over months and reproduces the fault as an intermittent one long after the invoice.
The places to look are the places the harness moves or has been disturbed: door and tailgate boots, engine harness connectors that flex on the mounts, splice packs, and previous repairs. A terminal that has spread or retracted inside its housing produces an identical fault to a cut wire and passes a visual inspection.
Treat the vehicle as not driveable while this is stored. Systems that depend on the network — stability control, transmission control, driver assistance — will be operating on defaults or not at all.
Common causes
- Broken CAN-Low conductor inside a door, tailgate, or engine harness boot that flexes in service
- Terminal retracted or spread inside a connector housing, indistinguishable from a cut wire without opening it
- Previous repair using untwisted wire, which removes noise immunity along that section
- Repair terminal of the wrong plating corroding against the original gold-plated contact
- Corroded splice pack, invisible without unwrapping the joint
- Harness re-routed alongside a power feed, ignition lead, or motor circuit after other work
- Water ingress into an inline connector corroding one pin of the pair
- Cut or pierced conductor from accessory fitment, alarm installation, or bodywork
- Rodent damage to a bulkhead-area harness
- Connector left unlatched after dash, door, or drivetrain work
Symptoms
- Multiple modules dropping in and out rather than failing cleanly and staying failed
- Faults that appear under electrical load — headlights, blower, wipers, or high alternator output — and clear at idle
- Network that tests fine in a workshop and fails on the road
- Several warning lamps illuminating together and then extinguishing
- Intermittent gauge or cluster dropouts while the engine keeps running
- Symptoms that change with road surface or with a door being opened and closed
- Communication faults that worsen in wet weather
- Stability control, transmission, or driver assistance systems operating on defaults
Diagnostic steps
- 1.Treat intermittency as evidence rather than an obstacle. An open on one conductor of a differential pair destroys noise cancellation, so a fault that comes and goes with electrical load is characteristic of this code rather than confusing.
- 2.Test with the vehicle loaded — headlights, blower, rear demister, wipers — rather than at idle in a quiet workshop, because that is the condition the fault needs.
- 3.Measure resistance across the pair at the diagnostic connector with the ignition off, and treat a reading near 120 ohms rather than 60 as evidence the bus is split and only one terminator is reachable.
- 4.Flex harness boots at the doors, tailgate, and engine mounts by hand while watching the module list or a meter, since fatigue breaks in these locations are the most common cause.
- 5.Open connectors along the suspect run and inspect for retracted or spread terminals, which pass a visual check from outside the housing.
- 6.Inspect every previous repair on the bus. Untwisted patch wire and mismatched terminal plating both reproduce this fault, the first immediately and the second months later.
- 7.Check that the harness still runs where it was designed to. A section re-routed next to a power feed or ignition lead loses the separation the design relied on.
- 8.Unwrap suspect splice packs rather than squeezing them, because internal corrosion passes a continuity check.
- 9.Repair with a short, crimped and sealed joint, keep the pair twisted through the repair, and use terminals of the same plating as the original.
- 10.Verify the repair with the vehicle under electrical load and on the road, not by the absence of a fault at idle.
Repair cost
$100 – $800
Diagnosis runs $150 to $300 and sits at the upper end more often on this code than on a hard failure, because the fault has to be reproduced under load before it can be located. A conductor or terminal repair is $150 to $500 depending on access. There is rarely a part to buy. The avoidable cost here is the second visit: a repair made with untwisted wire, or with a terminal of the wrong plating, returns the vehicle with a fault that behaves differently rather than one that has gone, and the diagnostic time is spent again from the beginning.
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.