U0073 — Control Module Communication Bus A Off
U0073 means the entire communication bus has shut down or been dragged down — not one missing module, but a network-wide failure, most often caused by a short in the wiring or a single faulty module corrupting the bus.
Severity: High · Can you keep driving? No. A bus that is down can disable starting and multiple safety systems at once; have the car towed to a shop equipped for network diagnosis.
What the U0073 code means
Where U0100 reports one missing voice, U0073 reports silence from the room. The CAN bus is a shared pair of twisted wires that every module talks on; the standard expects specific voltage patterns and a total resistance of about 60 ohms from the two terminating resistors. U0073 — control module communication bus 'A' off — sets when a module detects that the bus itself has stopped working: the dominant/recessive signaling has collapsed, usually because the wires are shorted together, shorted to power or ground, or one connected device is transmitting garbage or holding the bus down.
That 'one bad device' mechanism is the signature of this code. A single module with an internal short — after water damage, for instance — can take every other module offline with it, so the symptoms look catastrophic while the cause is one component or one chafed section of harness. Rodent damage and rub-through where the harness passes body panels are equally classic.
Expect chaos on the dash: multiple warning lights, gauges sweeping or dead, no-crank or crank-no-start, and a scan tool that may not communicate with anything at all through the diagnostic port. The totality is itself a clue — many unrelated systems do not fail simultaneously by coincidence.
Symptoms of U0073
- Many warning lights on at once, or a completely dead instrument cluster
- No-crank, or crank with no start
- Scan tool unable to communicate with any module
- Gauges dead, sweeping, or frozen
- Transmission, ABS, airbag, and power steering warnings together
- Symptoms that may come and go with bumps, steering movement, or wet weather if the cause is a chafed or wet harness
What causes U0073?
- Shorted CAN wiring — the two bus wires rubbed together, or one shorted to power or ground at a chafe point — door jambs, under seats, around the steering column, and where harnesses cross sharp body edges.
- One module dragging the bus down — a water-damaged or internally shorted module (a telematics unit, a flooded floor module, an aftermarket device) corrupts all traffic. Disconnecting modules one by one is how it is found.
- Water intrusion — flood cars, leaking windshields or sunroof drains, and wet connectors corrode pins and bridge circuits — a history of damp carpets is a real diagnostic clue.
- Rodent damage — chewed insulation in the engine bay or under seats shorts the twisted pair. Look for nesting material and droppings around the harness.
- Severe low voltage or a failed battery connection — modules drop off and error counters overflow during cranking with a dying battery, sometimes setting bus-off codes that vanish once voltage is restored and codes are cleared.
- Aftermarket electronics — remote starters, alarms, trackers, and OBD-port dongles spliced into the bus can take it down, especially budget installations with Scotch-lock taps.
How serious is U0073?
A downed bus can disable the starter interlock, fuel and spark control, ABS, airbag readiness, and electric power steering simultaneously. This is not a drive-it-and-see fault — the car may not start, or may lose systems in motion. Tow it.
Cost anxiety is understandable, but the most common causes are wiring and a single module, not a wholesale electronic failure. The expensive version of this repair is the one done by replacing modules at random; the reasonable version is a technician with a multimeter, a wiring diagram, and patience.
How much does it cost to fix U0073?
Labor-heavy diagnosis with a wide range of outcomes; the measurement steps, not parts-swapping, are the product. Typical US costs, parts and labor:
| Repair | Typical US cost (parts & labor) |
|---|---|
| Battery/cable correction (if voltage is the cause) | $100–$350 |
| Harness repair at a chafe or rodent-damage point | $300–$800 |
| Connector/module water-damage repair | $300–$900 |
| Replacement of the module dragging the bus down (incl. programming where needed) | $500–$1,500 |
Prices are typical US ranges and vary by vehicle, location, and shop. Diagnosis usually costs $100–$150 and is often applied to the repair bill if you proceed at the same shop.
How U0073 is diagnosed and fixed
- Verify battery condition, terminals, and cranking voltage first; a bus that only collapses during cranking on a dying battery is a battery job wearing a disguise.
- With the battery disconnected, measure CAN resistance at the diagnostic port: about 60 ohms is healthy; near 0 suggests wires shorted together, near 120 suggests one leg open, and other readings guide the search.
- Check bus voltages key-on: CAN-high plus CAN-low should average around 2.5 volts each in a complementary pattern; a line pinned at 0 or battery voltage identifies the short's direction.
- Isolate by division: disconnect modules (or split the bus at its junction connectors) one section at a time until communication returns — the last thing unplugged before recovery is the culprit branch or module.
- Inspect the implicated harness run closely for chafe, rodent damage, and water; check connectors for green corrosion and bent pins.
- Repair wiring with sealed splices or replace the failed module (with programming if required), clear all codes in every module, and verify full communication and a clean road test.
Frequently asked questions
What is the difference between U0073 and U0100?
U0100 means one module — the engine computer — is not answering, while the bus itself works. U0073 means the bus itself has shut down: signaling has collapsed, usually from shorted wiring or one bad module dragging the whole network down, so many modules go offline together.
How much does it cost to fix U0073?
Diagnosis is electrical and labor-based. Outcomes range from $100–$350 if voltage or cables are at fault, through $300–$800 for harness repair, to $500–$1,500 when a module must be replaced and programmed. Prices vary by vehicle, location, and shop.
Can I drive with U0073?
No. A bus-off fault can disable starting, steering assist, ABS, and airbag systems unpredictably. Have the car towed to a shop that does network diagnosis rather than attempting to drive it.
Can one bad module really take down the whole car?
Yes. Every module shares the same two wires. A module with an internal short can hold the bus in a state where no clean communication is possible, so everything looks broken until that one device is unplugged — which is exactly how technicians find it.
My scan tool will not connect to anything. Is that a clue?
It is one of the best clues. A scan tool talks over the same bus; if it cannot reach any module, the fault is bus-level (wiring, a dragging module, or power to the diagnostic port) rather than a single silent module. Tell the shop — it shapes the whole diagnostic path.
Related codes
More in this system: Network codes · Full codes list