U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module A

A generic network and communication trouble code, same meaning on every brand.

Fix now, drive carefully

Braking works, but assume ABS and stability control are offline - drive gently, brake early.

$120 - $1,500

typical repair range

NHTSA complaints mention this code

77,866

median mileage when reported, mi

Symptoms

ABS and traction lights on together
No ABS function
Speedometer issues on some cars

Likely causes, most common first

CAN wiring to the ABS module
ABS module power or ground
Failed ABS module

Read U0121 yourself before paying for diagnosis

A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.

What is going on and how it gets diagnosed

U0121 means the ABS control module dropped off the network. Base hydraulic braking survives - pedal to master cylinder needs no computer - but anti-lock, stability control and often traction control are gone until communication returns.

Module rebuild services exist for many popular ABS units at a fraction of new-part cost; worth asking about once diagnosis confirms the module rather than its wiring.

Repair options and cost

Network and power diagnosis$120 - $300

Wiring repair$150 - $800

ABS module repair or replacement$500 - $1,500

Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.

Owner reports mentioning U0121

“ABS systems not functioning properly, showing engine code U0121 that states lost communication with ABS module. This is a common occurrence with jeeps and is a safety issue due to the abs system not working properly. Also causes the brakes to randomly engage and disengage.”2008 Jeep Wrangler · Dec 2025

“ESP control unit software Recall 2016090002 Recall 2017110002 Recall 2020080016 And my car shows there's cero recall active but is giving me problem showing code: U0121 u1110 U0122 B227d U0423 B227d”2017 Mercedes-Benz C 300 · Mar 2025

“The problems listed in my vehicle are the following - U0073: Control Module Communication Bus A Off - U0100: Lost Communication With Engine Control Module - U0121: Lost Communication with Electronic Brake Control Module - U0140: Lost Communication With Body Control Module - P0700: Transmission Control Module Requested MIL Illumination I believe this is due to a bad design in the wiring. Also the electric brake vacuum broke which is a common problem.”2020 Chevrolet Malibu · Jul 2024

Show 7 more owner reports

“Codes c2209-49, c2200-47, u0121, u11bc, u1110, u1412. ABS, traction control, brake and CEL iluminated. No speedometer and no power steering. Codes will not clear. All wheelspeed sensors are operational per scantool in live data mode. Issue not confermed by dealer or independant as the cost is outrageous. And i cannot find a recall to repair issue. These issues i have had since March 2024. Cost for me to repair would be approximately $1200. Safety systems like these should not need a recall they've had years to resolve issues with ABS problems. I'm hoping that Mopar sees these issues with other people and has a recall on the problem. My wife won't even drive the truck.”2014 Ram 1500 · Mar 2024

“Vehicle will turn off while driving, causing steering wheel to lock up. This has happened 3 times. Engine light has come on this time and has codes U0100, U0121 and U11. I've had the vehicle checked at NTB, but was told these must be addressed by Chrysler. My safety has been affected while driving on the interstate and the vehicle shut down.”2012 Chrysler 200 · Dec 2022

“While driving the traction and stability lights came on along with the ESC service light. Also my Lane assist stopped working. It stayed on for at Least 30 mins until I turned off the car and restarted. Stopped at the closest garage and they checked the battery and used a scan tool to identify that it had a fault code U0121 Lost Communication with Anti-lock Brake System Module.”2018 Chevrolet Equinox · Oct 2021

“Error codes U1120, U1110, and U0121 all appeared at the same time. Car became undriveable while driving on the highway. Had to pull over with lost power and stop on the side of the highway. Tried to get the car started again and upon reignition the car cycled between air bag and transmission warnings. The car could not drive without reaching 7000+ RPMS at 5 mph. Ended up deserting the vehicle and had it towed to the nearest Jeep dealer. This is the 4th time the vehicle has broken down on the highway.”2014 Jeep Cherokee · Sep 2021

“Code u0121 lost connection with electronic brake control module. When coming to a stop, the vehicle stalls and the rpm's drop. Jerky acceleration. While driving at speed, vehicle stutters an acceleration.”2014 Chevrolet Malibu · May 2019 · 90,000 mi

“Steering froze up while leaving my driveway and p/s light came on. Shut engine off and back on, but did not resolve. I recall at least one other time that steering locked up temporarily while at a stop. Took to dealership for diagnostic as manual recommends. Diagnostic shows dtcs u0105 and u0121. Found tab st005-06 related to this issue and "updated power steering ecu available". Diagnostic $150, parts and labor $1600+ cleary this is a safety issue requiring a recall. Please investigate there are many more similar complaints.”2006 Toyota RAV4 · Jan 2019 · 125,600 mi

“Steering wheel jerking while in park and while driving to the point you had to put a lot of strength into keep car in lane, service esc: power steering warning light chimes and comes on while driving, steering wheel locks up while trying to pull over on the side of the road. I took the car to kipo chevrolet, the codes that came up were c0545(00), u0121, and c0661. Kipo chevrolet stated torque sensor readings were surging around not holding steady, would have to replace torque sensor and re-evaluate concern.”2009 Chevrolet Malibu · Feb 2018 · 130,358 mi

Cars that report U0121 most often

Based on mentions of this code in NHTSA owner complaints.

Frequently asked questions

Can I drive with U0121?

Braking works, but assume ABS and stability control are offline - drive gently, brake early.

How much does it cost to fix U0121?

Typical US repair costs range from $120 for the simplest fix (network and power diagnosis) to $1,500 in the worst case. The right number depends on which cause your car has - see the causes list.

Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.