U0121 on Honda: How to Diagnose & Fix
Lost Communication with ABS Module
1 Honda model · 1 documented issue · $150-$2,000 repair
U0121 on Honda by model: symptoms, diagnosis and fix
Some HR-V owners report the car will not start after it has been parked for a few days, sometimes again after the battery has already been replaced. Dealers have found nothing wrong, or put it down to the small factory battery. A 2019 owner was told to drive more and keep the car plugged into a battery maintainer. Causes owners found differ from car to car. On one 2016, a dealer replaced the battery and found nothing else, and the owner later got a diagnosis of a failing main ground cable; fixing it ended the problem. A 2022 owner traced the drain to two fuses that power circuits while the engine is off (the same fuses the shipping instructions say to pull), and the dealer replaced the battery under warranty. We found no Honda service bulletin for HR-V battery drain. Honda bulletin 17-032 covers a VSA-module drain, but it applies only to the 2017 CR-V. A separate fault, an internal failure of the VSA (stability control) modulator, makes the dash warning lights flash and stay on. One 2016 owner paid nearly $1,500 for a new modulator at a dealer. That is a different repair from a battery drain.
Common Symptoms
- Car will not start after sitting for a few days
- Battery goes flat again after being replaced
- Dealer battery and alternator tests find nothing wrong
- VSA and other dashboard warning lights flash and stay on (VSA modulator failure)
How to Fix
1) Rule out the simple causes first. Make sure the dome light is set to off, not door/auto. Keep the keyless fob away from the car when it is parked, because a nearby fob can keep talking to the car and drain both the fob and the car battery. Check that both battery cables and the main ground cable are clean and tight. A failing main ground was the cause on one 2016. 2) Have the battery and alternator tested, not just the voltage checked. If the battery is weak, replace it before looking for a drain. 3) If a good battery still goes flat, ask for a parasitic draw test. On one 2016, the dealer tested the battery and alternator but did not test for a draw. With the car off, draw should be under about 50 mA. Pulling fuses one at a time while watching the reading shows which circuit is drawing power, which is how a 2022 owner found two fuses that stay powered with the engine off. 4) Under the new-car warranty, a dealer may replace the battery, as on a 2022 HR-V. Keep records of each no-start. We found no HR-V recall or extended warranty for battery drain. Bulletin 17-032 (VSA software update for a 350 mA drain) covers only the 2017 CR-V, but you can ask the dealer to check your VIN for any open campaigns. 5) If the VSA and other dash warning lights flash and stay on, have the stability control system scanned for codes. If the VSA modulator itself has failed internally, it has to be replaced (one owner paid nearly $1,500 at a dealer).
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- DieHard Platinum 51R-AGM battery, 435 CCAOEMDieHard 51R-AGM
Replacement battery branch only after charging-system and battery testing confirms the battery is failed or cannot hold rated capacity Fitment/availability: BCI Group 51R application for 2016-2023 U.S. Honda HR-V 1.8L/2.0L; confirm fitme…
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Owner tips & cautions
- TipHRVForum DIY: Pull fuse #13 (power door locks) overnight - if battery stays charged, you have a bad door lock actuator causing the drain.
- WarningDon't just keep replacing the battery - find and fix the parasitic drain or you'll kill battery after battery.
- TipDisconnect the infotainment harness overnight (under dash) - if drain stops, the head unit is staying awake and needs replacement/reflash.
Sources for this issue (7)
- NHTSA Vehicle Complaints for 2016 Honda HR-Vnhtsa
- Reddit r/MechanicAdvice Thread Discussing Honda HR-V Battery Drain Issuesforum
- Car Talk Community: 2016 Honda HR-V batteryforum
- CarComplaints: 2019 Honda HR-V dead batteryowner-report
- CarComplaints: 2022 Honda HR-V weak batteryowner-report
- CarComplaints: 2016 Honda HR-V electronic stability controlowner-report
- Honda Service Bulletin 17-032: Parasitic battery draw from VSA modulatortsb
U0121 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does U0121 mean on a Honda?
U0121 stands for "Lost Communication with ABS Module." U0121 is a network communication code meaning a control module on the vehicle's data bus has lost communication with the ABS (anti-lock brake system) module. Modern cars share information over a CAN bus, and when other modules stop receiving expected messages from the ABS unit, this code sets. Because the ABS module also handles traction control and stability control on most vehicles, you may see multiple warning lights and lost features. It points to a communication or power problem with the ABS module rather than a worn brake part. On Honda it is documented across 1 model: HR-V.
What are the most common causes of U0121?
The most common causes of U0121 are: Loss of power or ground to the ABS module, Blown fuse feeding the ABS controller, Damaged, corroded, or loose ABS module connector, Open, shorted, or chafed CAN bus wiring, Internal failure of the ABS control module, Corrosion or water intrusion in connectors/harness, Faulty gateway module disrupting bus communication. The specific cause varies by vehicle.
How much does it cost to fix U0121 on a Honda?
Published repair estimates on Honda range from $150 to $2,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.