OBD-II trouble code
U0075: Control Module Communication Bus C Off
"Bus off" is not a description, it is a defined hardware state. A CAN controller counts its own transmit failures and takes itself off the network when the count passes 255 — which means the module reporting this is often the victim rather than the culprit.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $150 – $2,200
- DIY difficulty
- Shop recommended
Browse every code in U0001–U0075, or start from the full code library.
What does U0075 mean?
U0075 is set when a controller on the vehicle's third communication bus enters the bus-off state. That phrase is worth taking literally, because it refers to something specific rather than to a general opinion that the network is unwell.
Every CAN controller keeps two internal counters: one for errors it encounters while transmitting, one for errors while receiving. Successful traffic decrements them; failed traffic increments them, and transmit errors are weighted more heavily than receive errors. When the transmit error counter passes 255 the controller disconnects itself from the bus by hardware rule, stops driving the wires entirely, and reports it. Nobody decided that. It is a threshold defined in the CAN specification and enforced in silicon, which is why this code behaves so consistently across manufacturers when so much else about vehicle networks does not.
The most useful consequence of that mechanism is a diagnostic one, and it reverses the intuitive reading. A controller goes bus-off because its own transmissions kept failing — and transmissions fail when the wires are shorted, when the differential pair is unbalanced, or when another node on the same bus is corrupting frames. The module that stored the code is simply the one whose counter filled first. Replacing it is one of the more expensive ways to arrive back where you started. Treat the reporting module as a witness, not a suspect, until the wiring and the other nodes have been cleared.
The recovery behaviour produces a symptom pattern that is close to a signature. A controller that has gone bus-off will usually attempt to rejoin the network after observing a set quantity of idle bus time, and if the underlying fault is still there it fails again almost immediately. From the driver's seat that looks like systems that work for a few seconds, drop out, come back, and drop out again in a rhythm. Intermittent faults that fade in and out over minutes are usually connectors. A fault that cycles on a short, regular beat is usually a controller repeatedly attempting bus-off recovery, and that observation alone tells you what kind of fault you are chasing.
This is also the code where an oscilloscope stops being optional. A multimeter reads an average, and to an average reading a healthy bus, a bus full of error frames, and a bus with one node transmitting garbage all look broadly similar. On a scope they look nothing alike: you can see whether CAN High is being dragged toward ground, whether the differential swing has collapsed, and whether error frames are being generated by one node in particular. Attempting a bus-off diagnosis with a meter alone is how people end up disconnecting modules at random.
One thing follows from the letter, and it is about the vehicle rather than the fault. A car with a third bus has enough electronic content to justify splitting traffic three ways, and on many designs that third segment is where chassis, radar, camera and driver assistance modules live. So a Bus C failure disproportionately takes out lane keeping, adaptive cruise, blind spot monitoring and automatic braking rather than comfort features — and once the network fault is repaired, several of those systems may need a calibration procedure before they will operate again. Budget for that; it is frequently left out of the original quote.
Common causes
- CAN High or CAN Low on the Bus C pair shorted to ground, to battery voltage, or to each other
- A single node on the bus transmitting corrupted frames and driving every other controller's error counter up
- Open or high-resistance conductor in the bus pair from chafing, rodent damage or accident repair
- Failed or missing terminating resistor, collapsing the differential swing the controllers rely on
- Water in a connector or splice pack bridging the two bus conductors
- Corroded, backed-out or spread terminal at a module connector on the Bus C branch
- Poor ground at one of the modules, shifting its reference so its transmissions are read as errors
- Aftermarket equipment spliced into the bus pair — trackers, dash cameras, remote starters, trailer modules
Symptoms
- Systems that work for a few seconds, drop out, and return in a repeating cycle as the controller attempts bus-off recovery
- Driver assistance features disabled together — adaptive cruise, lane keeping, blind spot, automatic braking
- Multiple warning lights appearing at the same instant rather than one at a time
- Scan tool losing and regaining contact with the same group of modules
- Radar or camera modules reporting unavailable or blocked
- Warning messages that clear on a restart and return within a minute of driving
- Several U-series codes stored across different modules describing the same segment
- Symptoms strongly tied to road vibration, temperature or moisture
Diagnostic steps
- 1.Treat the module that reported the code as a witness rather than the cause. A controller goes bus-off because its transmissions kept failing, which usually implicates the wiring or another node.
- 2.Note whether the symptoms cycle on a short regular beat. That rhythm is bus-off recovery being attempted and failing, and it distinguishes this fault from an ordinary intermittent connection.
- 3.Get a wiring diagram and list every module on Bus C for this specific vehicle before testing anything, since bus assignments are not standardised.
- 4.Scope the bus pair rather than metering it. Error frames, a collapsed differential swing and a node transmitting garbage are indistinguishable on a multimeter and obvious on a waveform.
- 5.Check whether CAN High is being pulled toward ground or CAN Low toward supply, which localises the short to one conductor.
- 6.With the key off and the battery disconnected, measure across the pair and compare against the service data for that specific segment rather than assuming a standard value.
- 7.Disconnect nodes one at a time and watch whether the bus recovers and stays recovered, which identifies a jabbering module.
- 8.Inspect any aftermarket wiring tapped into the bus, which is over-represented in bus-off faults because splices are often made without regard to the differential pair.
- 9.Check the grounds of the modules on that segment, since a shifted ground reference makes a healthy transmitter look like a faulty one.
- 10.After repair, confirm the bus stays up through a full drive cycle, then check whether any driver assistance module on that segment now requires calibration.
Repair cost
$150 – $2,200
Diagnosis is $180 to $400, and it is worth paying for properly here because scope work is what separates this fault from guesswork. A wiring or connector repair on the bus pair is $150 to $650 depending on where the damage is. A terminating resistor or splice repair sits at the lower end. Replacing a module that is genuinely corrupting the bus, with programming, runs $400 to $1,100. The item people forget is calibration: because a third bus commonly carries radar, camera and driver assistance modules, restoring the network often has to be followed by a static or dynamic calibration at $200 to $600 before those systems will operate. European and premium vehicles with dense multi-bus architectures sit at the top of every figure here.
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. The barrier here is module programming, security access, and bus-level diagnosis that need manufacturer-grade scan tools and the credentials to use them. DIY attempts often produce a more expensive problem than the original code.