OBD-II trouble code
U0004: High Speed CAN Communication Bus (+) Low
CAN-High is being held down when it should be driven up. That inverts the differential into a state no receiver has a definition for, and transmitting modules detect it themselves within microseconds and take themselves off the network.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does U0004 mean?
On an idle high-speed CAN bus both conductors rest at about 2.5 volts. To send a dominant bit, a transmitting module drives CAN-High up toward 3.5 volts and CAN-Low down toward 1.5, and receivers read the roughly two-volt difference. CAN-High is therefore the conductor that is supposed to go up, and U0004 sets when it is being held down instead.
The electrical consequence is not simply a weaker signal. With CAN-High pulled toward ground and CAN-Low still resting near 2.5 volts, the difference between them is negative — CAN-High sits below CAN-Low, which is a condition the protocol does not define and receivers cannot resolve into either a dominant or a recessive bit. This is not a marginal signal; it is an invalid one.
What happens next is specific to this fault and is the most useful thing to know about it. Every CAN transmitter monitors the bus while it is sending, comparing each bit it puts out against what actually appears on the wires. A module trying to drive CAN-High up against a short to ground reads back a level that does not match what it sent, declares a bit error, and increments its transmit error counter — and that counter climbs eight at a time on errors while falling only one at a time on successes. A node crosses into the bus-off state within milliseconds of trying to talk. So the network does not degrade over minutes; the modules disqualify themselves almost immediately and stay off, because leaving bus-off requires a deliberate recovery sequence rather than a lucky retry. That is why this code so often arrives with a car that was fine one second and had a dashboard full of lights the next.
The second thing worth knowing is thermal, and it explains faults that appear to heal. A transceiver driving its output into a dead short to chassis is sourcing current continuously into ground. CAN transceivers include thermal shutdown for exactly this reason, so the chip protects itself, stops driving, cools, and tries again. The visible result is a network that comes back for a while after the car has been standing and dies again shortly after it is woken. That pattern is diagnostic rather than random.
The measurement that finds it is not the resistance across the pair — it is the resistance from CAN-High to chassis ground, taken with the battery disconnected. On a healthy vehicle that path should be high; near zero identifies the short directly. Better still, it gives you a stable value to divide against: disconnect modules and branch connectors one at a time, or in halves, and watch for the reading to return to normal. The branch that restores it contains the fault.
As for what causes it, a short to chassis has a different cause list from a short to power, because chassis is everywhere. Screws driven through a loom during accessory fitment, a harness pinched under a seat rail or a trim clip, a crushed section at a door boot, and water sitting in a connector with a grounded shell are the recurring ones.
Common causes
- CAN-High shorted to chassis ground where a screw or fastener has been driven through the loom during accessory or trim fitment
- Harness pinched under a seat rail, trim clip, or bracket
- Crushed or abraded loom section at a door, tailgate, or bulkhead pass-through
- Water standing in a connector with a grounded shell, bridging the pin to the body
- Internal short in a module pulling CAN-High down, which affects the whole bus from one branch
- Damaged terminal touching the connector body or a grounded bracket
- Corrosion inside a splice pack bridging CAN-High to a ground circuit
- Rodent damage exposing conductor against a metal panel
- Poor previous repair with unsealed joints resting against chassis metal
Symptoms
- Sudden and total loss of communication rather than a gradual degradation
- Dashboard filling with warning lamps within a second or two of the fault appearing
- Scan tool unable to establish communication with most or all modules
- Network that works briefly after the vehicle has stood, then fails again shortly after it wakes
- Repeated cycles of apparent recovery and failure that follow no obvious road or weather pattern
- No-start, crank-no-start, or immediate limp mode
- Instrument cluster dead or frozen while the engine may still run on defaults
- Fault first appearing after accessory installation, trim removal, or seat work
Diagnostic steps
- 1.Disconnect the battery and measure resistance from CAN-High to chassis ground. This is the measurement that identifies the fault; a reading near zero confirms a short to ground directly.
- 2.Do not rely on the resistance across CAN-High and CAN-Low. A short to chassis leaves both terminating resistors intact, so that reading can look entirely normal while the bus is unusable.
- 3.Use the CAN-High-to-ground reading as a stable reference and disconnect branches or modules in halves, watching for the value to return to normal. The branch that restores it contains the fault.
- 4.Interpret an apparent recovery after the vehicle has stood as thermal shutdown in a transceiver rather than as an intermittent wiring fault, and expect it to fail again once things warm up.
- 5.Establish what was fitted, removed, or disturbed recently. A fastener through a loom during accessory or trim work is a leading cause and the timing usually identifies it.
- 6.Inspect under seats, along sill trims, and at every point the harness passes a bracket or clip, since chassis metal is available to the loom almost everywhere.
- 7.Check connectors for standing water, particularly any with a metal shell or a grounded backshell.
- 8.Where disconnecting a specific module restores the reading, confirm by measuring at that module's connector before condemning it, since the fault may be in the branch rather than the module.
- 9.Repair the short before repeatedly powering the network up, because each attempt is another period of a transceiver sourcing current into ground.
- 10.After repair, confirm normal resistance from CAN-High to ground as well as normal resistance across the pair before declaring the job finished.
Repair cost
$100 – $900
Diagnosis is $150 to $300 and the resistance-to-ground measurement described above is what keeps it there — it converts a search of the whole vehicle into a halving exercise with a number to watch. A wiring or connector repair to clear the short is $150 to $600, with the spread reflecting how buried the contact point is; a screw through a loom under a seat is quick and a crushed section behind the dash is not. If the short is internal to a module, replacement with programming runs $400 to $900 or more and higher on European and luxury platforms. A transceiver that failed as a consequence of the short, rather than as its cause, is the reason not to keep re-energising the bus before the fault is cleared.
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.