U0073: control module communication bus off — the whole network went quiet

Most U-codes name a module that stopped answering. U0073 is different and more serious: the communication controller detected so many transmission errors that it took itself off the bus entirely.

Background: CAN bus fundamentals for car owners.

What “bus off” means

CAN controllers count errors. Every failed transmission increments a counter; every success decrements it. When the counter passes a threshold, the controller enters “bus off” and stops transmitting.

That is deliberate. A module transmitting corrupted messages continuously would jam the network for everything else, so the protocol includes a way for a faulty node to remove itself.

So U0073 means: something was going so wrong on the network that a controller gave up.

The symptom picture

Dramatic, and that is a clue in itself:

• Many warning lamps at once, across unrelated systems.
• A long list of U-codes in every module you can still reach.
• Instruments dead or erratic.
• The engine possibly not starting, or running in limp mode.
• A scan tool that cannot connect to some or all modules.

A dozen faults arriving simultaneously is one fault. The list of codes is the symptom, not the diagnosis.

When the scanner lies about ghost codes.

Causes, in order

1. Low or unstable system voltage

Check this first, always. A weak battery or a bad ground makes modules reset and transmit garbage, which is exactly what drives error counters up.

A remarkable share of catastrophic-looking network faults are a corroded battery terminal.

P0560 to P0563 system voltage faults and battery terminals corrosion.

2. One failed module dragging the bus down

A module with a shorted transceiver pulls the bus lines toward power or ground, and no one can communicate.

The diagnostic method is disconnection. Unplug modules one at a time and see when the network recovers. The module whose removal restores communication is the culprit.

That is a workshop procedure on most vehicles, because reaching the modules is the hard part.

3. Wiring

The bus is a twisted pair. Damage that matters:

• A short between CAN-High and CAN-Low.
• A short of either line to power or ground.
• A broken wire, which affects everything downstream.
• Water in a connector, bridging the two lines.

Water is worth emphasising. Connectors in wheel arches, under seats, and in the boot area are common entry points: damp carpet and electrical faults and water in the spare wheel well.

4. Termination resistance

A CAN bus is terminated at both ends with 120 Ω resistors, giving 60 Ω across the pair when measured with everything off.

That measurement is one of the most useful tests available: the 60 ohm test.

Reading	Meaning
~60 Ω	Both terminations present
~120 Ω	One termination missing — a module disconnected or failed
Near 0 Ω	Short between the lines
Very high	Break in the bus

5. Aftermarket installations

Alarms, trackers, dashcams and audio installs spliced into the bus are a recurring cause. Anything installed shortly before the fault appeared is a suspect: aftermarket accessories electrical faults and alarm and tracker installation.

What to do first, practically

1. Do not start replacing modules. The code list will make several look faulty.
2. Check battery voltage and terminals.
3. Check ground straps.
4. Ask what happened recently — installation work, water ingress, an accident, a jump start.
5. Measure bus termination with everything off.
6. Record every code before clearing anything.
7. Clear, then re-scan, and see which codes actually return.

That last step is important. Many of the U-codes will not come back once the supply is stable, and the short list that does return is the real diagnosis.

Related codes you will see alongside

U0100 lost communication with ECM, U0101 with TCM, U0121 with ABS and U0140 and U0155.

You can check this yourself. Pair an ELM327 adapter, open the free tier of the app and start with system voltage rather than the code list. On a network fault, the supply is the first thing to prove and the most common thing to find.