U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module A
What Does U0121 Mean?
U0121 is a diagnostic trouble code indicating: Lost Communication With Anti-Lock Brake System (ABS) Control Module A. This code relates to the network communication system. It has been reported in NHTSA complaints across 15 different vehicle models.
Symptoms of U0121
- ⚠️ Check engine light illuminated
- ⚠️ Engine misfires
- ⚠️ Vehicle won't start
- ⚠️ Jerking or surging while driving
- ⚠️ Traction control system activated
- ⚠️ ABS warning light on
Common Causes
- 🔍 Worn or fouled spark plugs
- 🔍 Clogged or leaking fuel injector
- 🔍 Wiring or connector problem
- 🔍 Intake manifold or gasket leak
- 🔍 Sensor or control module malfunction
U0121 Reports by Year
Real Owner Reports
From NHTSA complaint database — actual owner descriptions.
"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. "
— Toyota Rav4 owner, 01/30/2019
"HEARD ABNORMAL MECHANICAL RATTLE FROM THE FRONT END GOING OVER ROUGH STREETS IN SAN FRANCISCO. TOOK VEHICLE IN SUSPECTING SUSPENSION ISSUE TO CAPITOL TOYOTA OF SAN JOSE. DIAGNOSIS WAS STEERING RACK HAS A KNOWN PART ISSUE, INTERNAL TO ITS DESIGN, NOTHING THE DEALER COULD DO, AS FACTORY DID NOT ALLO"
— Toyota Rav4 owner, 05/25/2012
"SPORT MODEL FRONT END HAS HAD A LOUD POPPING/ CREAK NOISE WHEN GOING OVER BUMPS AT A ANGLE AND ALSO SHARP LOW SPEED MANUEVERS ( PULLING INTO TIGHT PARKING SPACE) SINCE IT WAS BASICLY BRAND NEW. LOUD TICKING NOISE FROM FRONT HUBS AT LOW SPEEDS FIRST VISIT TO FORD WAS AT 9,000 MILES THEY COULD NOT DUP"
— Ford Explorer owner, 10/26/2016
Source: NHTSA Complaints Database
Most Reported Vehicles for U0121
Based on NHTSA owner complaint data.
| # | Vehicle | Reports |
|---|---|---|
| 1 | CHEVROLET MALIBU | |
| 2 | TOYOTA RAV4 | |
| 3 | DODGE CHARGER | |
| 4 | FORD EXPLORER | |
| 5 | FORD FOCUS | |
| 6 | CHEVROLET EQUINOX | |
| 7 | CHEVROLET CRUZE | |
| 8 | CHEVROLET TRAVERSE | |
| 9 | CHEVROLET COLORADO | |
| 10 | JEEP CHEROKEE | |
| 11 | JEEP LIBERTY | |
| 12 | JEEP GLADIATOR | |
| 13 | VOLKSWAGEN GTI | |
| 14 | MERCEDES-BENZ C-CLASS | |
| 15 | CHRYSLER 200 |
U0121 Reports by Vehicle
Detailed NHTSA complaint analysis for each vehicle model.
Frequently Asked Questions
What causes communication codes?
Can a bad battery cause network codes?
Reported Repair Costs for U0121
Based on 7 owner-reported repair costs from NHTSA complaints.
⚠️ These are owner-reported costs, not estimates. Actual costs vary by location, labor rates, and root cause. Source: NHTSA complaints database.
What To Do Next
Possible Fixes
- 🔧 Have the code scanned with a professional-grade scanner to read freeze frame data
- 🔧 Test the relevant sensor(s) with a multimeter before replacing
- 🔧 Inspect wiring harness and connectors for damage, corrosion, or loose connections
- 🔧 Check battery voltage and ground connections; inspect CAN bus wiring
- 🔧 If DIY repair isn't feasible, get a diagnostic from a trusted mechanic ($50-150)
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1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
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2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
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3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.