U0121 Lost communication with the anti-lock brake (ABS) module

Lost communication with ABS control module

U0121 in short

U0121 means other modules lost contact with the anti-lock brake module. The usual causes are wiring, connectors and power or ground at the ABS module; some GM codes are a software glitch. Normal brakes should still work but anti-lock and stability control may not, so have it checked soon by a professional.

The ABS module shares wheel speed and brake data with the engine computer, cluster and stability systems. When other modules stop hearing it, they store U0121 and usually turn on the ABS and stability warning lights.

The ABS module sits low in the engine bay, so its harness and connector see water, heat and rubbing. GM's PIT-4756 found the harness to the brake module chafing on the transmission mount on 2007-2009 Equinox, Torrent and Vue.

Not every U0121 is a real fault. GM says U0121 stored in history on some 2013 Malibu, Regal and LaCrosse and on the 2017 XT5 is a software anomaly that needs no parts. Brakes are safety-critical, so the repair belongs with a professional.

Most likely causes

  1. Damaged ABS module harness, connector or network wiring

    Common general cause
    Why it ranks here
    Network faults are usually wiring. GM's PIT-4756 traced U0121 with ABS and StabiliTrak lights to the brake module harness chafing on the transmission mount or a bracket.
    What would change this
    Rises if the ABS module does not answer a scan tool or the harness shows rubbing or corrosion. Falls if the module communicates and wiring is intact.

  2. ABS module power, ground or fuse fault, or low battery voltage

    Common general cause
    Why it ranks here
    A module without power cannot talk. ABS modules have their own high-current fuses and grounds.
    What would change this
    Rises if the ABS module does not respond and its fuses or grounds test bad. Falls if supply is good.

  3. History code from a software anomaly with no real fault (some GM models)

    Supported by vehicle data
    Why it ranks here
    GM says U0121 in history on the 2013 Malibu, Regal and LaCrosse multi-axis sensor and on the 2017 XT5 is a software anomaly; no parts are needed.
    What would change this
    Rises if U0121 is history-only with no warning lights on a covered model. Falls if the ABS light is on or the code is current.

  4. Failed ABS module electronics

    Possible — needs testing
    Why it ranks here
    Possible once wiring, power and bulletins are ruled out, but less common than wiring faults.
    What would change this
    Rises if the module has good power, ground and bus wiring and still will not communicate.
    How to test the brake system control module (abs module)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Have all modules scanned, noting whether U0121 is current or history and whether the ABS module answers the scan tool.

    Low riskTool: scan tool that reads all modules

  2. Have a shop check the ABS module fuses, power and ground, and inspect its harness for chafing, then measure the network wiring.

    Best left to a shopTool: multimeter, wiring diagram

    Warning:Brakes are safety-critical. ABS repairs may need bleeding with a scan tool. Leave this to a professional.

Symptoms

  • ABS warning light on
  • Traction or stability control warning, or stop/start not working
  • Several warning lights at once
  • Red brake warning light on with the ABS light, or a brake pedal that feels wrong Gets more serious: stop and have it checked.

How to fix

This is a safety-critical system, so we explain the code but don't give repair steps. Have it inspected by a qualified technician.

Parts and cost context

  • ABS module:An ABS module replacement often needs programming and brake bleeding with a scan tool. Rule out wiring and power first. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Chevrolet

  • 2007–2009 · Equinox

    Supported by vehicle data

    GM PIT-4756: U0121 with ABS and StabiliTrak lights from the brake module harness chafing on the transmission mount (also Torrent and Vue). Whether it is still current is not verified.

  • PIT-4756Status unverified

    ABS and StabiliTrak lights on with U0121 and C-codes

    2007–2009equinox · issued · NHTSA 10028186

    The harness to the brake module can chafe on the transmission mount or a metal bracket. Also covers Pontiac Torrent and Saturn Vue.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Cadillac

  • 2017 · XT5

    Supported by vehicle data

    GM 16-NA-177: U0121 sets from a software anomaly and does not affect the car. Whether it is still current is not verified.

  • 16-NA-177Status unverified

    U0121 sets due to a software anomaly

    2017xt5 · issued · NHTSA 10116601

    Informational: U0121 sets from a software anomaly, does not turn on the MIL and does not change vehicle performance.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Buick

  • 2013 · Regal

    Supported by vehicle data

    GM PIC-5652-B: U0121 in the multi-axis sensor during a scan needs no parts (also LaCrosse, Malibu). Whether it is still current is not verified.

  • PIC-5652-BStatus unverified

    U0121 stored in the multi-axis acceleration sensor module

    2013lacrosse, regal · issued · NHTSA 10054262

    The code is set during a technician's DTC check and no components need replacing. Also 2013 Malibu.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Jeep

  • 2018–2022 · Wrangler

    Supported by vehicle data

    FCA S2208000025 lists the ABS lamp, U0121 and stop/start inoperative with a LIN bus code. Whether it is still current is not verified.

  • S2208000025Status unverified

    ABS lamp on, U0121, stop/start inoperative

    2018–2022wrangler · issued · NHTSA 10222946

    Service information for a Wrangler with the ABS lamp on, U0121 and U0037 (LIN bus) and stop/start not working.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • U0415A module received invalid data from the anti-lock brake (ABS) module

    Same system Possible — needs testing

    U0121 means the ABS module went silent; U0415 means it is talking but sending data another module rejects. Both point at the ABS module and its wiring.

  • U0100Lost communication with the engine or powertrain computer

    Shares causes Supported by vehicle data

    GM's GMLAN bulletin lists U0121 with U0100 and other lost-communication codes from one backed-out terminal.

All Module network (CAN bus) communication codes

U0121 questions

Do my brakes still work with U0121?

Normal braking usually remains, but anti-lock and stability control may not work. Honda's manual says the car keeps normal braking without anti-lock when the ABS light is on. If the red brake light also comes on, stop and get help.

Sources and review

  1. 2023 Integra Owner's Manual: Indicators — American Honda Motor Co.
  2. GM PIT-4756: ABS and StabiliTrak lights on with U0121, C0550 and others — General Motors / NHTSA ,
  3. FCA S2208000025: ABS lamp on, U0121 lost communication with ABS module, stop/start inoperative — FCA US LLC / NHTSA ,
  4. 2023 Integra Owner's Manual: Indicator, Coming On/Blinking — American Honda Motor Co.
  5. ASE L1 study guide: Network U-codes — Open Exam Prep
  6. GM PIC-5652-B: U0121 stored in the multi-axis acceleration sensor module — General Motors / NHTSA ,
  7. GM 16-NA-177: U0121 sets due to a software anomaly — General Motors / NHTSA ,
  8. FCA 21-043-25: TCM update for U0415-00 — FCA US LLC / NHTSA ,
  9. GM 08-07-30-021: Loss of high-speed GMLAN, intermittent no crank, DTCs U0073/U0100/U0101/U0102/U0121/U0140 — General Motors / NHTSA ,
Last reviewed
Written by
Saurabh K.
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