Electrical system and control unitsGeneric OBD-II codeUrgency: Garage soon
Code U0101: loss of communication with the gearbox control unit
U0101 means: loss of communication with the transmission control module (TCM).[1][2][3] The gearbox control unit (TCM) has stopped communicating with other modules in the car, usually over the CAN network. According to AutoDTCs, the engine control unit stops receiving the CAN messages it expects from the TCM and, because the two modules work very closely on gear changes and torque management, the gearbox switches to an emergency operating mode.[1][2]
What code U0101 means
The gearbox control unit (TCM) has stopped communicating with other modules in the car, usually over the CAN network. According to AutoDTCs, the engine control unit stops receiving the CAN messages it expects from the TCM and, because the two modules work very closely on gear changes and torque management, the gearbox switches to an emergency operating mode.[1][2]
TroubleCodes.net clarifies that the communication may have been lost now or in the past, and that the fault may be between two modules or in the CAN bus itself. If the TCM does not know the positions of the solenoids, the gearbox may stay in default positions.[3]
It is a communication fault between modules, not a fault of a gearbox sensor. It is a generic network code (it starts with U0), so it means the same on any brand.[2][4]
Original definition (SAE J2012): Lost Communication With Transmission Control Module (TCM)[1][2][3]
Causes of U0101
From most to least likely, according to the sources.
- 1CAN network wiring (CAN high or CAN low) open, broken or short-circuited, often chafed along the body rails or near hot areas; also damaged cables in the gearbox area (even by rodents, according to TroubleCodes.net).[1][2][3]
- 2TCM power fuse or relay blown or failing (TroubleCodes.net cites the IOD fuse with poor contact).[1][3]
- 3TCM connector corroded, loose or with bent terminals.[1][3]
- 4Low battery voltage or poor body or engine earth, which degrade the CAN signal.[1][3]
- 5Faulty gearbox control unit: an internal power fault, an internal short circuit or a CAN transceiver fault.[1][2][3]
Symptoms
- Engine fault warning light on and, sometimes, the gearbox warning light on.[1][2][3]Engine warning light: faults that cause it
- Limp mode: the gearbox stays in one gear (usually 2nd or 3rd) and does not shift.[1][2][3]Limp mode: faults that cause it
- Abrupt engagement when selecting D or reverse, and a torque converter that does not lock, with high revs on the motorway (AutoDTCs).[1]
- Loss of cruise control and, as a side effect, ABS, traction or stability warning lights.[1]Electronic warnings: faults that cause it
- Gear selector position indicator (PRNDL) inaccurate or off, or diagnostic equipment that does not power up (TroubleCodes.net).[3]
How to diagnose U0101
In order: from the most likely and cheapest to check to the most expensive.
- 1Check the technical bulletins and look for other communication or battery codes: diagnose those first.[1][2][3]
- 2Check the fuses and relays that power the TCM and that their earths are firm. Measure the battery voltage (engine off and running) and inspect the main earth.[1][2][3]
- 3Inspect the TCM connector and harness: corrosion, moisture, bent or protruding terminals, chafed or gnawed cables. Re-seat the connector and protect the joints with dielectric grease.[1][3]
- 4Measure the resistance of the CAN network at the diagnostic connector, between pins 6 and 14: about 60 ohms is normal. About 120 ohms suggests a missing terminating resistor or one half of the bus disconnected, and infinite resistance a complete break. Move the wiring while measuring to find the fault.[1][3]
- 5Check that the TCM receives 12 V and a good earth at its connector. If the TCM responds to the diagnostic equipment, the fault is often intermittent: clear the codes, carry out a road test and read again.[2][3]
- 6If the engine control unit has been replaced, verify the VIN stored in the module. Only if everything else is good, consider the TCM; its replacement usually requires programming, so TroubleCodes.net recommends a workshop or dealership.[3]
Can I keep driving with U0101?
Garage soon
AutoDTCs rates it as high severity: the gearbox is left in limp mode or without shifting, a tool that reads all the modules is needed to diagnose it, and its advice is to drive only to the workshop and not to tow the car until it has been diagnosed. CódigosDTC and TroubleCodes.net agree that the gearbox may be left in 2nd or 3rd gear. The prudent course is to avoid long journeys.[1][2][3]
Parts related to code U0101
The parts that most often cause this code. The code points to the circuit, not always the part: diagnose before you buy.
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Documented faults related to U0101
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.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Main relay of the power relay assembly (overheating and fire risk)Yes, on the first units: Hyundai and Kia recalled 2016–2018 Ioniq and Niro hybrids and plug-in hybrids because the main relay that connects the high-voltage battery could come loose, overheat and, in the worst case, cause a fire.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Clutch hydraulic actuator (HCA): leak or moisture with risk of short circuit and fireYes, this is the most documented hybrid system fault: the hydraulic actuator that engages the combustion engine with the hybrid assembly has had two recalls (2018 for a leak on the Niro and 2025 for moisture in its electronic board on the Niro, Ceed and XCeed PHEV), with a risk of short circuit and fire.
- 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.
- 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
- High severityFrequency: CommonHyundai-Kia E-GMP (Ioniq 5, Ioniq 6, EV6)Faulty ICCU: 12 V battery drains, charging fails and power is lostThe ICCU, which combines the on-board charger and the converter that powers the 12 V system, can fail on the Ioniq 5, Ioniq 6 and EV6: it cuts AC charging, drains the 12 V battery and, in the worst case, the car loses power and stops. Hyundai and Kia have recalled units built from 2021 to 2024.Repair: €2,000–€5,000
- Medium severityFrequency: Very frequent1.5 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.
- Medium severityFrequency: Very frequent1.8 HybridThe 12V battery drains on the C-HR and Corolla hybridsYes: on the first-generation C-HR hybrid, the 12V battery is one of the most frequently reported faults: it runs flat after a few days standing or on short trips, and the car will not start. The ADAC and Toyota itself point to it as a cause of faults on the C-HR and Corolla.
- Medium severityFrequency: Very frequent2.0 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.
- 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.
- High severityFrequency: OccasionalFord Galaxy IIIAcid leak from the 12 V battery (campaign 18P11)According to the fiches-auto.fr page, campaign 18P11 (August 2019) covers Galaxies built between February 2014 and February 2019 over a risk of battery acid leaking, which can worsen over time.
Automatic gearbox (torque converter and CVT): the most serious faults by engine and car
- Critical severityFrequency: CommonMultitronicWear of the variator chain and conesYes: the variator uses a chain of more than 1,000 links between two pairs of cones that wears with mileage, especially if the oil is not changed regularly. Gebrauchtwagenberater ties the gearbox's reputation to "numerous total failures"; Schaeffler/LuK sells a kit to repair the chain.
- Critical severityFrequency: Common2.0 CDI (OM640)Costly failure of the Autotronic (CVT) automatic gearboxYes: the Autotronic continuously variable gearbox, optional on the 180 CDI and 200 CDI, is the problem cited most by owners of the 140 hp version (24 of 114 testimonies) and a fault hotspot according to the ADAC up to 2006. A German source recommends avoiding it and choosing the manual gearbox.
- High severityFrequency: CommonMultitronicControl unit (TCU) failure and limp modeYes: the multitronic control unit, especially in the 01J code (up to 2007), is one of its best-documented weak points. Overheating with oil seeping into the control unit causes fault warnings and limp mode and, if ignored, can end up damaging the gearbox itself.
- High severityFrequency: Common7G-Tronic / 9G-TronicConductor plate and speed sensor failure (limp mode)Yes: one of the most cited weak points of the 7G-Tronic (722.9) is the internal conductor plate, which integrates the turbine, output and intermediate-shaft speed sensors; if it fails it can trigger a limp mode with delayed shifts or harsh jerks, although it does not always require replacing the whole gearbox.
- High severityFrequency: CommonAudi A6 C6Wear and failures of the Multitronic automatic gearbox on front-wheel-drive versionsYes: on front-wheel-drive versions with the Multitronic (CVT) gearbox, gebrauchtwagenberater.de documents jerking when pulling away that can progress to complete failure, especially on older cars; fiches-auto.fr adds selector lock-ups and torque converter breakages. The quattro versions, with the Tiptronic gearbox, do not share this problem.Repair (garage): €1,000Appears at around 120,000 km
- High severityFrequency: CommonMINI Cooper R50Faults in the CVT automatic gearbox, the manual gearbox and the clutchAccording to the reports collected by fiches-auto.fr, the Mini R50 accumulates 22 gearbox mentions and 32 clutch mentions, and the CVT automatic variant is described as fragile and 'to be avoided'. Wikipedia confirms that the CVT and the manual gearboxes were fitted depending on the year.
Frequently asked questions
What does U0101 mean?
That the automatic gearbox control unit has stopped communicating with the rest of the modules over the CAN network. The code does not say whether the fault is in the wiring, the module's power supply or the module itself.[1][2][3]
Why does the gearbox stay in one gear with U0101?
Because without communication with the TCM, the gearbox goes into limp mode. CódigosDTC and TroubleCodes.net cite it staying in 2nd or 3rd gear, or in limp mode.[2][3]
Can a weak battery cause U0101?
Yes. AutoDTCs and TroubleCodes.net include low battery voltage among the causes, and TroubleCodes.net says it can cause a false U0101, so it is worth measuring it before continuing.[1][3]
Can I keep driving with U0101?
AutoDTCs rates it as high severity and recommends driving only in limp mode to the workshop, without towing the car until it has been diagnosed.[1]
Will I pass the MOT with code U0101?
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.[5]
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.
- U0101 - Lost Communication With TCMAutoDTCs · Technical documentation · consulted 31 October 2026
- U0101 - Comunicación perdida con el módulo de control de transmisión (TCM)CódigosDTC · Technical documentation · consulted 31 October 2026 · in Spanish
- U0101 trouble code - Data bus: transmission control module (TCM) no communicationTroubleCodes.net · Technical documentation · consulted 31 October 2026
- On-board diagnosticsWikipedia · Other source · consulted 31 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