Electrical system and control unitsGeneric OBD-II codeUrgency: Garage soon
Code U0073: communication bus "A" off (CAN)
U0073 means: control module communication bus "A" off (bus off).[1][2] It is a network code: the car's modules (engine, gearbox, ABS, body...) stop communicating over the CAN bus, intermittently or completely. CarParts points out that it can appear in the engine module, the gearbox module, the ABS or the body module.[1]
What code U0073 means
It is a network code: the car's modules (engine, gearbox, ABS, body...) stop communicating over the CAN bus, intermittently or completely. CarParts points out that it can appear in the engine module, the gearbox module, the ABS or the body module.[1]
DTCSearch lists it as a generic network code and, for Mazda, with the definition "CAN data bus off", with wiring, the CAN bus or the engine ECU as causes. In the CAN standard, a node enters the "bus off" state when its transmit error counter exceeds 255 and it stops transmitting frames.[2][3]
Original definition (SAE J2012): Control Module Communication Bus "A" Off[1][2]
Causes of U0073
From most to least likely, according to the sources.
Symptoms
- Engine warning light on.[1]Engine warning light: faults that cause it
- Loss of engine power.[1]Less power: faults that cause it
- Poorer fuel economy.[1]Uses more fuel: faults that cause it
- Possible starting failure: the engine does not turn over or start.[1]Hard to start: faults that cause it
- Warning message on the instrument cluster.[1]Electronic warnings: faults that cause it
- The automatic gearbox may not change gear.[1]
How to diagnose U0073
In order: from the most likely and cheapest to check to the most expensive.
- 1Check the battery condition and the connections of the CAN bus unit: low voltage causes communication faults.[1]
- 2Read all the codes stored in all the modules and compare them with the manufacturer's; they help show which modules have lost communication.[1]
- 3Visually inspect the CAN bus area for broken wires or loose connectors and measure the continuity of the bus "A" modules.[1]
- 4Consult the manufacturer's technical service bulletins (TSB). CarParts considers the diagnosis complicated and recommends leaving it to a professional.[1]
- 5Replace the faulty module only at the end; some modules need programming after replacement.[1]
Can I keep driving with U0073?
Garage soon
CarParts does not rate the severity, but cites everything from poorer fuel economy to a car that will not start or a gearbox that will not change gear; the impact depends on which modules lose communication.[1]
Documented faults related to U0073
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Control units and comfort electronics (BSI, SAM...): the most serious faults by engine and car
- High severityFrequency: CommonPeugeot 408Electronic faults with the doors, boot and screen (suspected BSI control unit)In 2025 Peugeot recalled around 60,000 2023-2024 Peugeot 308 and 408 cars (code MUV) over a software fault in the door control unit that can cause unintended opening or closing and disable the anti-trap protection on the windows; 408 owners also describe a power boot lid that will not open or close and a screen that goes black.
- High severityFrequency: CommonRenault Laguna IIMultiplex electrical system and hands-free card faultsThe Laguna II had a reputation problem because of faults in its multiplexed electrical network: the hands-free card, the card reader and the ABS, ESP and handbrake warning lights came on erratically, especially in the early years of production (2001-2003).
- Critical severityFrequency: OccasionalMazda MX-5 IVDC/DC converter control unit fault with risk of short circuit and fireA 2018 Safety Gate alert covers MX-5 ND models built between August 2014 and April 2017: the DC converter control unit could fail, cause a short circuit and damage the converter, with a risk of fire. It is a precautionary recall; check the chassis number at the official service centre.
- Medium severityFrequency: Very frequentFiat StiloElectrical and electronic faults in the multiplex systemThe Stilo has had a reputation for widespread electrical faults since its launch: poor contacts in the multiplex wiring, an instrument cluster that switches off and sensors that disconnect, sometimes while the car is running.
Wiring, connectors and short circuits: the most serious faults by engine and car
- Critical severityFrequency: OccasionalBMW N62 / N63Additional turbo coolant pump with a risk of short circuit and fireIn 2011 BMW recalled 5, 6 and 7 Series, X5 and X6 models built between 2007 and 2011 because of the additional turbo coolant pump: moisture, a short circuit and, in extreme cases, a fire. In 2012 there was another campaign for a crack in the pump, with a more robust version.
- High severityFrequency: Occasional1.3 TCeBlown fuel pump fuseOn the Renault Captur II and Clio V with the 1.3 TCe engine, a 20-amp fuse can blow and leave the car without an engine. According to L'argus, in April 2021 this fault still had no factory fix; the official service centre's diagnosis includes checking, and sometimes replacing, the catalytic converter.
- High severityFrequency: OccasionalM264/M254Poorly tightened 48 V earth wiring in the mild-hybrid system (fire risk)Mercedes-Benz has issued several recalls (2022-2025) because the 12 V and 48 V earth connections of some cars with a 48 V system may be poorly tightened: resistance rises, the connection heats up and a fire cannot be ruled out. The official workshop checks them and repairs them if necessary in an hour or more.
- High severityFrequency: Occasional1.9 TDIUnit injector tappet wear and injector wiring damageOn the unit injector system, the tappets that operate the injectors can wear if the oil is not changed on time, and the injector wiring can be damaged by contact with oil, causing misfires or loss of power.
- Critical severityFrequency: CommonRover 75Seat SRS connector fault with airbag and seatbelt warning light onAuto Express warns that the Rover 75's seat SRS connector can cause the airbags to fail, and dealers were fixing it with revised connectors. The Dutch APK inspection puts the airbag or seatbelt fault warning among the most frequent defects across almost all series.
- Critical severityFrequency: OccasionalAlfa Romeo GiuliettaShort circuit from badly routed wiring that can prevent gear shifting (KBA campaign, 2010-2015)Yes: according to Germany's KBA, on Alfa Romeo Giulietta cars built between 2010 and 2015 inadequate cable routing could cause a short circuit and, as a result, gear-shift failure.
12 V battery: the most serious faults by engine and car
- Medium severityFrequency: Common2.0 and 2.5 HybridThe 12 V battery goes flat if the car sits unused for several daysYes, it is a common complaint: the 12 V battery can go flat after just a few days without using the car, because of the standby drain from the electronics (multimedia, telematics, keyless entry). With the 12 V battery very low, the hybrid system can lock itself into a safe mode.
- Medium severityFrequency: OccasionalE-Tech Híbrido12 V battery going flat or needing early replacementSeveral Clio, Arkana and Austral owners with the E-Tech engine document premature replacement of the 12 V auxiliary battery, in one case twice in the first year. It is not the most widespread fault of the system, but it appears repeatedly across different models.
- Medium severityFrequency: Occasional1.5 Hybrid12 V battery going flat while parkedYes: on the hybrid Yaris and Yaris Cross, the 12 V battery can go flat within a few days while parked because of the residual draw of modules such as eCall or keyless entry. It is fixed with a software update or, if the battery is already damaged, by replacing it.
- Low severityFrequency: OccasionalHonda e:HEV / i-MMD12 V battery going flat and hybrid system failing to startIf the car stands unused for a long time, the 12 V auxiliary battery can go flat and the hybrid system does not activate ("Ready" does not appear) even though the traction battery is charged. It is fixed by charging or replacing the battery.Repair (garage): €120–€250
- High severityFrequency: CommonHyundai Ioniq 5Faulty ICCU: 12 V battery drain and loss of drive powerThe Integrated Charging Control Unit (ICCU) can fail on the Ioniq 5: it drains the 12 V battery, may refuse to charge and, in the most serious cases, cause the car to lose power or not go into drive mode. Hyundai has replaced this part and updated its software on units built between January and February 2023.
- High severityFrequency: OccasionalBMW Serie 6 E63/E64Battery drain and loose positive cableThe battery and alternator are frequently cited by Fiches-auto, and there is a recall for the battery's positive cable, which can work loose.
Frequently asked questions
What does code U0073 mean?
That communication between modules over data bus "A" has been lost, intermittently or completely.[1][2]
Is U0073 serious?
It depends on which modules are cut off: CarParts cites everything from higher consumption to the car not starting. Get it diagnosed soon.[1]
What causes it?
Damaged wiring or poor connections, a failed module (such as the engine ECU) or a data network problem, according to CarParts and DTCSearch.[1][2]
Can I fix it myself?
You can check the battery, read other codes and visually inspect wires and connectors, but CarParts recommends a professional for the full diagnosis.[1]
Will I pass the MOT with code U0073?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[4]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- U0073 Code: Control Module Communication Bus "A" OffCarParts.com · Technical documentation · consulted 29 October 2026
- U0073 DTC: Control Module Communication Bus "A" OffDTCSearch.com (OBD Solutions) · Technical documentation · consulted 29 October 2026
- CAN busWikipedia · Other source · consulted 29 October 2026
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish