U0073
NetworkControl Module Communication Bus "A" Off
The main data bus that lets your car's control modules talk to each other has shut down, so the ECU has lost contact with the network. Modern cars run almost everything over this CAN network, so when it goes quiet you don't get one tidy fault, you get a dashboard full of lights and several systems dropping out at once. For the owner this usually points at a wiring or connector problem somewhere on the bus, or a single module that's gone faulty and dragged the whole network down with it. It's a proper diagnostic job rather than a quick part swap.
ⓘ Information only. This page provides general educational information about fault code U0073. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does U0073 mean?
U0073 is a Network (CAN bus, module communications) fault code. It indicates: Control Module Communication Bus "A" Off.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • A cluster of warning lights coming on together, often ABS, traction control, airbag and engine light all at once
- • ABS, stability control or cruise control becoming unavailable
- • Instrument cluster going blank, freezing, or showing nonsense readings
- • Electric windows, central locking or other body electrics working one minute and dead the next
- • Car refusing to start, or starting then cutting out
- • Lights or other electrics flickering for no obvious reason
Possible causes
Causes commonly associated with U0073, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Damaged, chafed or corroded CAN bus wiring, the most common cause, often where a harness rubs through near the engine, behind the dash, or under the carpet where water has got in
- 2. A water-damaged or loose connector, particularly in door looms, under-bonnet plugs and connectors near the bulkhead that take vibration and moisture
- 3. A weak or failing battery, low or dropping voltage upsets the modules and can throw a string of communication codes including this one
- 4. A failed control module pulling the bus down, the engine ECU, body control module or ABS unit going faulty can stop the whole network
- 5. Poor or corroded earth connections on one or more modules, a bad earth makes the signals unreliable
- 6. Aftermarket kit wired badly into the network, dashcams, alarms, stereos and tow bar electrics spliced into the CAN lines can knock communication out
- 7. A software or firmware fault in a module, rarer but it happens, usually needing a dealer-level update
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Test and load-test the battery and check the charging voltage first. A flat or tired battery is the cheapest cause and you want it ruled out before you chase wiring for hours
- 2. Scan the whole car, not just the engine ECU, and note every U-code that comes up alongside U0073. The pattern of which modules report it tells you roughly where on the bus the break is
- 3. Check and clean every main earth point and the battery terminals. A poor earth mimics a wiring fault and costs nothing to put right
- 4. Go over the CAN wiring by hand, looking for chafing, green corrosion or water ingress, and pay attention to harness sections that flex or sit low where damp collects
- 5. Back-probe the CAN-high and CAN-low lines with a multimeter. A healthy bus reads around 60 ohms across the two lines at rest, roughly 120 ohms on each terminating resistor; readings well off that point to a wiring break or a duff module
- 6. If the wiring checks out, disconnect modules one at a time on the network to find the one that's killing the bus, then verify it before you commit to a costly replacement and coding
Common questions about U0073
How do I work out which of these is actually causing it on my car? +
Let the codes and the symptoms guide you. Start by testing the battery, because a tired one throws communication faults and it's a five minute check. If the battery's fine, scan the whole vehicle and look at which modules are reporting U-codes. If several scattered modules complain, you're likely looking at a main wiring or earth fault on the bus. If it's always the same single module dropping offline while everything else is happy, that module or its dedicated plug is the suspect. Wiggle-testing the loom while watching live data often flushes out an intermittent connector.
Can I sort this myself without a garage? +
Some of it, yes. Cleaning the earth points, checking the battery terminals for corrosion, load-testing the battery and having a look for an obvious chafed or water-soaked connector are all fair game at home. What you can't easily do is trace a CAN fault to a specific module or replace and code one, which needs a scan tool that reads the whole network and the know-how to interpret the bus voltages. If a quick battery and earth check doesn't clear it, hand it to an auto electrician or a good independent.
If I just clear the code, will it stay gone? +
Only if the underlying fault was something momentary like a flat battery during a jump start. Clear it on a car with a chafed wire or a failing module and it'll be back the moment the bus drops out again, often within minutes of driving. Clearing the code is fine as a test to see whether it returns, but it isn't a repair. Fix the wiring, earth or module first, then clear it and confirm it stays clear over a few drives.
What's the risk if I just keep driving with it? +
Real risk, so don't sit on this one. With the bus down you can lose ABS and stability control, meaning no anti-lock braking and no traction help if you have to stop or corner hard. The car may also refuse to restart once you switch it off, leaving you stranded. On top of that an intermittent network fault can drop the engine into limp mode or cut out while you're moving. Get it diagnosed promptly, and if systems are dropping out as you drive, recovery is the sensible call rather than risking a longer trip.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →