OBD-II trouble code
U0005: High Speed CAN Communication Bus (+) High
CAN-High is pinned above its normal range, which drives the bus differential permanently to the dominant state. That is not a degraded network — it is a jammed one, and the practical consequence is that no module can transmit, including the one you are trying to read the code from.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $120 – $1,400
- DIY difficulty
- Shop recommended
What does U0005 mean?
A high-speed CAN bus carries data as the difference between two wires rather than as a voltage on either one. Both sit near 2.5 volts when nothing is being sent. To send a dominant bit, a transmitting node pushes CAN-High up toward roughly 3.5 volts and pulls CAN-Low down toward roughly 1.5 volts, and every receiver on the bus reads the gap between them. U0005 is stored when a module observes CAN-High sitting well above where it belongs.
Why that specific fault behaves the way it does comes straight from the arithmetic. The receiver reads high-minus-low. Hold CAN-High up at battery voltage and the difference is not merely wrong, it is enormous and permanently positive — which every node on the network reads as a dominant bit that never ends. CAN arbitration is built on the rule that a dominant bit always wins, so a bus held dominant is a bus in which nothing can ever be transmitted by anybody. This is the reason U0005 behaves less like a communication fault and more like a switch being thrown. Modules do not drop off one at a time; the entire segment goes silent at once.
That has a consequence for how you approach the car, and it is the single most useful thing to know about this code. The standard opening move on a network fault is to scan every module and see who is missing. On a jammed bus that move may be impossible — the diagnostic tool talks over the same wires, so it cannot reach anything on the affected segment either. You may only be able to read U0005 from a module on a different sub-network through the gateway, or not at all until the short is cleared. Start with a meter at the diagnostic connector rather than with a scan tool, because on this fault the hardware measurement is available when the software one is not.
There is a second trap worth naming. The habitual health check on a CAN network is to measure about 60 ohms across the two bus wires with the battery disconnected, confirming the two 120 ohm terminating resistors. That test is looking for open circuits and missing terminators, and a short from CAN-High to a power source does not disturb either of them. A network jammed hard enough to be completely dead can still measure a textbook 60 ohms. Passing that check does not clear this fault, and a technician who treats it as a clearance will spend a long time looking in the wrong place.
The damage question matters more here than on most codes. Bus transceivers are built with some tolerance for a short to battery, but that tolerance is a survival rating rather than an operating one, and the longer battery voltage sits on the bus the more of them are being cooked. A vehicle that has been driven or repeatedly cranked with this fault present can end up needing more than the wiring repair that caused it, so after the short is cleared every module on the segment should be verified rather than assumed good.
On the physical side, the wire is the suspect and not the module. CAN-High and CAN-Low are twisted together deliberately, and that twist is what makes the pair reject electrical noise. The places they stop being twisted are the places faults happen: at splices, at connector bodies, and at the short stubs that branch off to individual modules. Those are also the places a chafed conductor meets a power feed running through the same bundle.
Common causes
- CAN-High shorted to battery voltage or to a switched power feed running in the same bundle
- Chafed conductor at a point where the pair leaves its twist — a splice, connector body, or module stub
- Water or electrolyte bridging CAN-High to a powered terminal inside a connector
- Collision or repair damage crushing the harness against a powered circuit
- An accessory or aftermarket installation spliced into the wrong wire
- A failed transceiver inside one module holding its output high
- Corrosion creating a resistive leak from a power circuit onto the bus
- Battery voltage applied to the bus by a jump start or charger connected incorrectly
Symptoms
- Complete loss of communication across an entire network segment at once, rather than one module at a time
- Scan tool unable to establish any connection on the affected bus
- Multiple warning lamps illuminated together, including ABS, stability control, and airbag
- Engine that cranks but will not start, or will not crank at all
- Instrument cluster dead, frozen, or showing default values
- Symptoms that appear instantly and completely rather than building over time
- Additional modules found damaged after the short is repaired
Diagnostic steps
- 1.Do not start with the scan tool. On a jammed bus the tool communicates over the same wires and cannot reach anything, so begin with a meter at the diagnostic connector.
- 2.Measure CAN-High to ground with the key on. A reading well above the 2.5 volt resting level, and steady rather than fluctuating with traffic, confirms the line is being held rather than driven.
- 3.Measure the differential between the two bus wires. A large steady positive difference indicates a bus held dominant, which distinguishes this from an open circuit where both wires sit near 2.5 volts or float.
- 4.Do not treat a 60 ohm resistance reading as a clearance. The terminating resistors are unaffected by a short to power, so a completely jammed bus can still measure normally.
- 5.Identify which power circuit is involved by checking whether the fault is present with the key off, on accessory, and on run. A fault that only appears in one key position points at a specific switched feed.
- 6.Trace the harness where the twisted pair passes near heavy power feeds, and inspect every splice, connector body, and module stub, since those are the points where the pair is untwisted and vulnerable.
- 7.Disconnect modules one at a time while watching CAN-High. If the line returns to normal when a particular module is unplugged, that module's transceiver is holding it high.
- 8.Look for recent electrical accessory work. An aftermarket device spliced into the wrong conductor produces this exact fault and is often the most recent change to the vehicle.
- 9.After the short is cleared, verify communication with every module on the segment individually. Prolonged battery voltage on the bus damages transceivers, and a second failure hiding behind the first is common.
Repair cost
$120 – $1,400
Diagnosis is $150 to $350 and is genuinely harder here than on most network codes, because the usual tooling cannot reach the bus and the fault has to be found with a meter and a wiring diagram. Repairing the short itself is often modest — $150 to $600 — since it is usually one damaged conductor. The variable that decides the final bill is how long the vehicle ran with battery voltage on the bus: transceivers survive it for a while and then do not, so module replacement and programming at $400 to $1,200 or more is a real possibility on a car that was driven for weeks with the fault present. That is a strong argument for towing rather than driving this one in.
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.