Electrical system and control unitsGeneric OBD-II codeUrgency: As soon as possible
Code U0100: lost communication with the engine control unit
U0100 means: lost communication with engine or powertrain control module (ECM/PCM) "A".[1][2] The engine control unit (ECM/PCM) and other control units in the car have stopped communicating with each other over the CAN data network, which all modules use to pass information to one another. Sources describe it from slightly different angles: Carista says the other modules no longer "see" the engine control unit on the network, and AutoDTCs says the engine control unit stops receiving the messages it expected from another module.[1][2][3][4]
What code U0100 means
The engine control unit (ECM/PCM) and other control units in the car have stopped communicating with each other over the CAN data network, which all modules use to pass information to one another. Sources describe it from slightly different angles: Carista says the other modules no longer "see" the engine control unit on the network, and AutoDTCs says the engine control unit stops receiving the messages it expected from another module.[1][2][3][4]
This is a communication fault between modules, not a sensor fault. That is why the effects range from none at all to the car not starting, according to CarParts.com, and it is common for several warning lights to come on at once, as Carista notes.[2][3]
The initial U indicates a communication network code and the 0 that follows indicates a generic code (defined by SAE), so it means the same on any make.[5]
Original definition (SAE J2012): Lost Communication With ECM/PCM "A"[1][2]
Causes of U0100
From most to least likely, according to the sources.
- 1Corroded, loose or damaged connectors on the module or the CAN network. AutoDTCs lists this as the most common cause.[1][4]
- 2Damaged, cut or short-circuited CAN network wiring (for example, with chafed insulation).[1][2][3][4]
- 3Faulty engine control unit (ECM/PCM): an internal fault, or a failure that short-circuits the network and cuts off communication for all modules.[1][2][3][4]
- 4No power or earth at the control unit: blown fuse or relay, corroded earth or damaged main wiring harness, or low battery voltage.[1][3][4]
- 5Control unit software error (corrupted firmware).[1][3]
- 6Faulty CAN network termination resistor.[1]
- 7Water or moisture in electrical components.[1]
Symptoms
- Engine warning light permanently on, often together with other warning lights on the dashboard.[1][2][3]Engine warning light: faults that cause it
- Hard starting or no start at all. CarParts.com adds that there may also be no symptoms.[1][2][3][4]Hard to start: faults that cause it
- The car goes into limp mode, with reduced power.[1][3][4]Limp mode: faults that cause it
- Several warning lights at once, erratic instrument cluster gauges, and dependent functions (such as cruise control) that stop working.[3][4]Electronic warnings: faults that cause it
- Loss of power and poor acceleration.[1]Less power: faults that cause it
- Degraded automatic gearbox, ABS and stability control, and an uneven idle.[1][3]
- The diagnostic tool cannot connect to the engine control unit, and other U codes from other control units may appear.[1][4]
How to diagnose U0100
In order: from the most likely and cheapest to check to the most expensive.
- 1Read the codes from all control units with a professional diagnostic tool, which lets you see which module is not responding, and note down any other communication (U) codes that appear.[1][4]
- 2Measure the battery voltage: according to AutoDTCs, it should be between 12.4 and 12.6 V with the engine off and between 13.8 and 14.4 V with the engine running. Also check the fuses (and relays) that supply the engine control unit.[1][3][4]
- 3Check that the control unit receives a 12 V supply and a solid earth at its connector; clean or repair any corroded earths.[1][3]
- 4Inspect the connectors and wiring harness of the CAN network for corrosion, loose contacts, and chafed or cut wires.[1][4]
- 5Measure the continuity (and, on the CAN line, the resistance) of the CAN high and low wires up to the control unit connector. This is a workshop test: CarParts.com notes that it is done with equipment such as an oscilloscope.[1][2][3]
- 6Ask the manufacturer whether a software update is available for the control unit.[1]
- 7Only if the power supply, earth and network are all fine and the control unit still does not respond, consider replacing it (and reprogramming it).[1][3]
Can I keep driving with U0100?
As soon as possible
The sources agree that this is serious. Autodoc Club does not recommend continuing to drive, because lost communication with the engine control unit can cause unpredictable behaviour, loss of power or the engine cutting out. Carista rates it as high severity and says the car may refuse to start or end up in a very restricted limp mode, to the point of being unusable in most cases. AutoDTCs recommends taking it to a workshop if it goes into limp mode or will not start, and CarParts.com says not to ignore the code and to have it diagnosed immediately, while pointing out that sometimes there are no symptoms. The sensible course is to stop using the car normally and take it to a workshop, or call a recovery truck, as soon as possible.[1][2][3][4]
Documented faults related to U0100
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
Can I drive with code U0100?
It is not advisable. Autodoc Club warns of possible unpredictable engine behaviour, loss of power or a stall, and Carista and AutoDTCs describe a car that may not start or may go into limp mode. The best course is to take it to a workshop as soon as possible, by recovery truck if necessary.[1][3][4]
Does U0100 mean the engine control unit is broken?
Not necessarily. Although a faulty control unit is one of the causes, the sources also list corroded or loose connectors, damaged CAN network wiring, blown fuses or relays, poor earths and low battery voltage. AutoDTCs names the connectors as the most common cause, and all of those checks are cheaper than replacing the control unit.[1][3][4]
Why do other U codes appear alongside U0100?
Because what is failing is the communication between modules. Autodoc Club lists other U codes from other control units among the symptoms, and Carista mentions several warning lights on at once and degraded gearbox, ABS and stability control. The codes that accompany U0100 help the workshop see which module is not responding.[1][3]
Can I diagnose U0100 myself?
Only the basic checks: according to CarParts.com, looking at the fuses and wiring is the limit of what a non-professional should do, and finding which module is not responding requires a professional diagnostic tool, as AutoDTCs states. Measuring the CAN network with an oscilloscope is workshop work.[2][4]
Does U0100 mean the same on all makes?
Yes. Codes that start with U0 are generic network codes defined by SAE, and they mean the same on any make.[5]
Will I pass the MOT with code U0100?
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.[6]
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.
- Código de avería U0100Autodoc Club · Technical documentation · consulted 4 October 2026 · in Spanish
- U0100 Code: Lost Communication with ECM/PCM "A"CarParts.com · Technical documentation · consulted 4 October 2026
- U0100 OBD-II codeCarista · Technical documentation · consulted 4 October 2026
- U0100 Code: Lost Communication With ECM/PCMAutoDTCs · Technical documentation · consulted 4 October 2026
- On-board diagnosticsWikipedia · Other source · consulted 4 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