The main CAN network has shut down or one module is dragging it down, so the control units can no longer talk to each other. You usually get a dash full of warning lights at once, not just the engine light.
U0073 — also written UOO73, U 0073, 0073 — is the OBD-II code for control module communication bus 'a' off. Same code, same fix: this page covers every spelling of it.
U0073 is a electrical fault code, typically costing £80–£900 to put right in a UK garage.
What causes U0073?
- Water in a connector or module (often under a seat or in the scuttle)
- Chafed CAN wiring shorting to earth
- A failed module pulling the bus down
- Flat or failing battery causing low voltage
- Poorly fitted aftermarket tracker, stereo or dash cam spliced into the loom
How U0073 is fixed
- Test and charge the battery before anything else
- Measure CAN resistance at the diagnostic plug (about 60 ohms is healthy)
- Unplug modules one at a time to find the one killing the bus
- Dry, clean and reseal wet connectors
- Remove any spliced-in aftermarket kit
U0073 repair cost in the UK
Most U0073 repairs land between £80–£900 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Can you drive with U0073?
Not advised. Gearbox, ABS and power steering can all drop out, so get it recovered or driven a short distance with care.
Urgency: high.
What U0073 is telling you
Electrical faults of this type are proved with a meter, not a parts catalogue. Supply, signal and earth are each tested at the connector before anything is ordered.
On U0073 specifically — control module communication bus 'a' off — the stored code records the moment that check failed, not the moment the part gave up. That gap is why two cars with the same code can need completely different repairs.
How U0073 is diagnosed properly
Each of the known causes is ruled in or out in cost order, cheapest and most likely first. On U0073 that means working through them like this:
- Water in a connector or module (often under a seat or in the scuttle) — opened, inspected for green corrosion and spread pins, and back-probed rather than unplugged, so the reading is taken with the circuit live.
- Chafed CAN wiring shorting to earth — checked with the loom flexed and the engine warm, because a wiring fault that is intermittent when cold shows itself under heat and vibration.
- A failed module pulling the bus down — checked for outstanding manufacturer updates before any hardware is touched.
- Flat or failing battery causing low voltage — confirmed with live data before any part is bought.
- Poorly fitted aftermarket tracker, stereo or dash cam spliced into the loom — checked with the loom flexed and the engine warm, because a wiring fault that is intermittent when cold shows itself under heat and vibration.
An hour of diagnostic time on U0073 routinely saves more than it costs, because the alternative is buying the most expensive part on that list and hoping.
The repairs that actually clear U0073
- Test and charge the battery before anything else
- Measure CAN resistance at the diagnostic plug (about 60 ohms is healthy)
- Unplug modules one at a time to find the one killing the bus
- Dry, clean and reseal wet connectors
- Remove any spliced-in aftermarket kit
After the repair the code is cleared and the car is driven until the relevant readiness monitor completes, rather than handed back as soon as the light goes out. A code that has only been erased will come straight back on the next drive cycle.
What happens if U0073 is ignored
Leaving this one is the expensive choice. The protection strategy the ECU falls into is there to limit damage, and every mile driven with the fault active is a mile the engine or gearbox spends outside its designed operating window.
For reference, U0073 typically lands at £80–£900 all-in at an independent garage in the UK — main-dealer quotes for the same job usually sit above that.
Getting U0073 looked at
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If U0073 is showing on your car, bring the scan results with you — we will read the live data ourselves, tell you which of the causes above it actually is, and price the repair before any work starts.