Control Module Communication Bus A Off

What U0073 means

Every module on a Controller Area Network (CAN) constantly watches the two-wire differential bus and keeps internal transmit and receive error counters. Each corrupted or acknowledged-but-failed message bumps the transmit error counter up, and clean traffic winds it back down. When that counter climbs past the controller's threshold (around 255), the CAN hardware declares itself 'bus off' and electrically disconnects its transceiver so a babbling or mis-wired node cannot jam the whole network. U0073 is the code the module logs when this happens on the bus it labels 'A' — typically the primary high-speed CAN. Because the trigger is excessive physical-layer errors, the fault points at the shared wiring, splices, connectors, the two 120 Ω terminating resistors, or a module dragging the bus down, rather than at one named component.

Symptoms

• Several warning lamps illuminate together — check-engine, ABS, traction/stability, and often airbag or power-steering lights — because many modules lose their shared data at once
• A scan reveals U0073 stored alongside a cluster of 'lost communication' U-codes (U0100-series) set by other modules that can no longer hear the affected controller
• The scan tool intermittently drops or cannot establish a connection with one or more modules on that bus segment
• Reduced-power or limp-home operation, erratic gauges, or frozen infotainment as modules fall back to safe defaults when live bus data disappears
• Hard-start, no-start, or stalling if the module that went bus-off carries messages the engine controller needs to run
• The fault often comes and goes with vibration, temperature, or steering/suspension movement when a chafed wire or loose terminal is the cause

Common causes

• An intermittent open, short-to-ground, or short-to-voltage in the CAN-A twisted pair — chafed, pinched, or broken harness wiring, frequently near a rub point or a spot disturbed by prior repair work (most common trigger)
• Corroded, backed-out, spread, or loose terminals at a module connector, an inline splice pack, or the diagnostic link connector on that segment
• A failed or out-of-spec terminating resistor, or damaged bus termination, letting reflections and noise corrupt messages until the counter trips
• Water intrusion or corrosion in a connector or module that periodically shorts the two bus wires together or to an adjacent circuit
• A single failing module on the bus that transmits malformed frames or holds a line low, forcing a neighbour (or itself) into bus-off
• Poor module power or ground — a high-resistance ground or dropping supply voltage that corrupts the module's transceiver output
• Aftermarket accessories, trailer wiring, or amplifiers spliced into the CAN or its power/grounds and injecting electrical noise

Severity & driving advice

Severity: High — A bus-off fault silences part of the data network, so multiple systems can lose function at once and warning lights and drivability problems are common.

Can I drive? Not recommended. When a module goes bus-off, safety systems (ABS, stability control, airbags) and even the engine controller can lose the data they need, so have it diagnosed before extended driving.

Diagnostic approach

1. Record all modules and all codes first — Scan every module, not just the powertrain, and write down which controllers report U0073 and which report lost-communication codes. The module that is missing from the others' point of view — or common to the failed segment — points you at the physical location of the bus break.
2. Inspect the harness and connectors on that segment — With the ignition off, wiggle-test the CAN-A wiring and unplug the suspect connectors. Look for chafe-through, green corrosion, spread or pushed-back terminals, and repair-tape splices. Physical-layer damage is the leading cause of a bus-off, so this visual/tactile check finds most faults.
3. Measure bus termination resistance — Key off and battery-safe, measure resistance across CAN-High and CAN-Low at the diagnostic connector. A healthy high-speed CAN with its two 120 Ω terminators in parallel reads roughly 60 Ω. About 120 Ω means one terminator or a branch is open; near 0 Ω means the pair is shorted together; infinite means an open bus.
4. Check CAN-High and CAN-Low for shorts — With modules disconnected as needed, verify neither bus wire is shorted to ground, to battery voltage, or to the other line. Any of these hard shorts will drive error counters up instantly and force the nearest controllers bus-off.
5. Verify power and ground at the offending module — Confirm the module reporting U0073 has clean battery-plus and a low-resistance ground (voltage drop well under 0.1 V on the ground path). A weak supply or corroded ground makes the transceiver output marginal and generates bus errors even when the wiring looks fine.
6. Isolate a bad node by disconnection — If the wiring checks good, unplug modules on the segment one at a time (or split the bus at a splice) and re-scan. If the bus recovers and U0073 stops setting when a particular module is removed, that module or its branch wiring is the fault.
7. Scope the differential signal if available — Where an oscilloscope is available, capture CAN-High and CAN-Low; healthy recessive lines sit near 2.5 V and swing to roughly 3.5 V / 1.5 V dominant. Ragged edges, missing swing, or one line stuck flat confirm which wire or node is corrupting traffic.
8. Repair, clear, and retest — Fix the wiring, terminal, resistor, or module you identified, clear codes across all modules, then road-test and re-scan to confirm no module returns to bus-off and the lost-communication codes stay cleared.

FAQ