Electrical system and control unitsGeneric OBD-II codeUrgency: Garage soon
Code U0121: lost communication with the ABS module
U0121 means: loss of communication with the anti-lock braking system (ABS) control module.[1][2][3] Communication over the CAN network between the ABS control unit and the car's other control units has been interrupted: Carista sums it up as the network no longer "seeing" the ABS module. According to AutoDTCs, that module sends out the wheel speed data needed by traction control, the gearbox and stability control, so without its messages those functions stop working.[1][2][3][4]
What code U0121 means
Communication over the CAN network between the ABS control unit and the car's other control units has been interrupted: Carista sums it up as the network no longer "seeing" the ABS module. According to AutoDTCs, that module sends out the wheel speed data needed by traction control, the gearbox and stability control, so without its messages those functions stop working.[1][2][3][4]
The sources agree on what is lost, namely the ABS, traction control and stability control (ESP), and on the fact that the normal braking system keeps working: Carista and AutoDTCs point out that conventional hydraulic braking remains. Autodoc Club does warn, however, that without those safety systems the car is less safe under emergency braking or in difficult conditions.[1][2][4]
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 thing on any make.[5]
Original definition (SAE J2012): Lost Communication With Anti-Lock Brake System (ABS) Control Module[1][2][3]
Causes of U0121
From most to least likely, according to the sources.
- 1Corroded, loose or damaged ABS module connector. AutoDTCs names this as the most common cause, especially on modules mounted in the engine bay or in the wheel arch.[1][2]
- 2Damaged, severed or short-circuited CAN network wiring between the ABS module and the rest of the network.[1][2][4]
- 3Faulty ABS control unit, due to an internal fault, a power supply fault or a fault in its CAN transceiver.[1][2][4]
- 4Blown fuse or relay in the ABS power supply, or a corroded or poor earth.[1][2][4]
- 5Battery voltage too low (AutoDTCs quotes a threshold of 9 to 10 V).[2]
- 6Faulty CAN network termination resistor.[1]
- 7Software error in the ABS module.[1]
Symptoms
- ABS, traction control and stability control warning lights permanently on.[1][2][4]ABS/ESP light: faults that cause it
- The ABS, traction control and stability control are disabled, and the braking aids stop working; normal braking remains operational.[1][2][4]
- Speedometer that reads incorrectly or not at all, because the engine control unit is not receiving the wheel speed.[1][2][4]
- The engine warning light may also come on.[2]Engine warning light: faults that cause it
- Other control units may also log communication errors.[1]
How to diagnose U0121
In order: from the most likely and cheapest to check to the most expensive.
- 1Scan all the control units to confirm that the ABS one is not responding and see which other communication codes accompany it. You need professional diagnostic equipment capable of analysing the CAN network; a basic OBD reader is not enough.[1][2]
- 2Measure the battery voltage: according to AutoDTCs, it should be between 12.4 and 12.6 V at rest and between 13.8 and 14.4 V with the engine running.[2]
- 3Check the ABS fuses and relay, and that the module is receiving power and earth.[1][2][4]
- 4Inspect the ABS module connector (corrosion, loose contacts), clean the connectors and check the continuity of the CAN wires up to that connector.[1][2][4]
- 5Check the earth points and the CAN network termination resistor.[1]
- 6Ask the manufacturer whether there is a software update for the ABS module.[1]
- 7Only if the module still does not respond even though the power supply and the network are fine, replace it and reprogram it.[1][4]
Can I keep driving with U0121?
Garage soon
The sources agree on the essentials, with some nuances. Autodoc Club says you can generally keep driving, but that without ABS and ESP the car can be dangerous under emergency braking or in difficult conditions, and recommends repairing it as soon as possible. Carista rates it as high severity, but notes that the car remains drivable because normal hydraulic braking works. AutoDTCs warns that braking performance is worse in emergency stops and on slippery surfaces, and advises driving carefully and repairing it soon, especially in bad weather.[1][2][4]
Documented faults related to U0121
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
ABS and stability control (ESP): the most serious faults by engine and car
- Critical severityFrequency: Very frequentHyundai Tucson IIIFire risk from the ABS control module (HECU)Yes. Hyundai recalled the Tucson TL built between February 2017 and August 2020 (campaign 11DC03) because the ABS module can fail internally and cause a short circuit with a risk of fire in the engine bay, even with the car parked. In the US the same campaign (20V543000) covered model years 2016-2021.
- Critical severityFrequency: Very frequentKia Sportage IVShort circuit in the ABS/ESP unit with fire riskYes: Germany's KBA and the US NHTSA have recorded several campaigns (2021 and 2025) over a short circuit in the ABS/ESP unit that can cause a fire in the engine bay, even with the car parked and switched off.
- Critical severityFrequency: Very frequentOpel Corsa FBrake fluid leak and loss of ABS/ESP from the EBS valve block (2024 production)The manufacturer recalled Opel Corsa F cars built between 24 May and 27 August 2024 because of an EBS valve block with a casting defect that can cause an internal brake fluid leak and the loss of stability control, ABS and electronic brake distribution.
- Critical severityFrequency: Very frequentPeugeot 2008 IIReduced braking effectiveness from an HBC electronic fault (2023 production)On Peugeot 2008 II models built between 2 October and 15 November 2023, an error in the vacuum circuit can make the electronic unit send an invalid value to the hydraulic brake compensation (HBC), which reduces braking effectiveness.
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.
Frequently asked questions
Can I drive with code U0121?
Generally yes, but with caution and by getting it repaired soon. The normal brakes keep working, according to Carista and AutoDTCs, but the ABS, traction control and ESP do not, and Autodoc Club warns that this can be dangerous under emergency braking or in difficult conditions.[1][2][4]
Do I still have brakes with U0121?
Yes: Carista and AutoDTCs state that conventional hydraulic braking remains. What is lost are the electronic aids (ABS, traction control, stability control), and AutoDTCs warns that braking performance gets worse in emergency stops and on slippery surfaces.[2][4]
Why does the speedometer fail with U0121?
Because the ABS control unit is the one that sends out the wheel speed data. If the engine control unit does not receive those messages, the speedometer may read incorrectly or not at all, according to Autodoc Club, Carista and AutoDTCs.[1][2][4]
Can I diagnose U0121 with a basic OBD reader?
Only to see the code. Autodoc Club states that diagnosis requires analysing the CAN network with professional equipment and that a basic OBD reader is not enough, and AutoDTCs recommends scanning all the control units to confirm that the ABS one is not responding.[1][2]
Does U0121 mean the same on all makes?
Yes. Codes starting 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 U0121?
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 U0121Autodoc Club · Technical documentation · consulted 4 October 2026 · in Spanish
- U0121: Lost Communication with Anti-Lock Brake ModuleAutoDTCs · Technical documentation · consulted 4 October 2026
- U0121 DTC CodeDTCSearch · Other source · consulted 4 October 2026
- U0121 OBD-II codeCarista · 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