U0121: Lost Communication with ABS Control Module
The CAN messages from the ABS module are missing. ABS, ESP/ESC and traction control all become inactive (warning lamps illuminate), the speedometer may go to zero on cars that derive vehicle speed from the ABS module, and adaptive cruise control is disabled.
What U0121 means
U0121 is a generic OBD-II network code in the lost communication group. In plain English: a control module stopped receiving messages from the ABS Control Module on the vehicle’s data network.
- Check engine light
- Usually on
- Emissions-related
- No
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
Post this code once and get quotes from local mechanics. Free to post.
Makes the repair and parts links below specific to your car. Kept in this browser only.
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
- 01ABS module unpowered (often a corroded ground or fuse)Most likely
- 02CAN wiring to ABS brokenPossible
- 03ABS module failedPossible
Can I drive with U0121?
Drive carefully and get it checked soon.
Normal hydraulic braking usually still works, but ABS, stability control or brake assist may be off. Leave extra stopping distance, especially on wet or icy roads. If the pedal feels soft or the red brake light is on, stop driving.
Why we rate it urgent
- ABS or brake assist may be disabled
- Turns on the check engine light
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose U0121
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just U0121. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Fix other faults first
If other codes are stored, start with power-supply, sensor-circuit and communication codes. They can cause this code as a side effect.
- 3
Check module power, ground and the data bus
Confirm the module named in the code has battery voltage, ignition feed and a good ground. With the battery disconnected, a healthy CAN bus usually measures about 60 ohms between CAN-High and CAN-Low at the OBD port. Inspect connectors for corrosion and pushed-out pins.
- 4
Test the likely causes in order
Start with the most likely cause and work down: ABS module unpowered (often a corroded ground or fuse); CAN wiring to ABS broken; ABS module failed. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
U0121 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Hard
- Good DIY job?
- Usually a shop job
Involved testing or major parts. Most owners use a shop.
Parts and systems involved
- ABS control module
- CAN bus wiring
U0121 FAQ
What does U0121 mean?
U0121 means “Lost Communication with ABS Control Module”. The CAN messages from the ABS module are missing. ABS, ESP/ESC and traction control all become inactive (warning lamps illuminate), the speedometer may go to zero on cars that derive vehicle speed from the ABS module, and adaptive cruise control is disabled.
Is U0121 serious? Can I keep driving?
We rate U0121 as urgent severity. Drive carefully and get it checked soon. Normal hydraulic braking usually still works, but ABS, stability control or brake assist may be off. Leave extra stopping distance, especially on wet or icy roads. If the pedal feels soft or the red brake light is on, stop driving.
What causes U0121?
The most common causes, from most to least likely: ABS module unpowered (often a corroded ground or fuse); CAN wiring to ABS broken; ABS module failed. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix U0121?
Typical labor is about $125–$750 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix U0121 myself?
DIY difficulty is hard. Most owners leave this to a shop because it needs involved testing or specialist tools. Always confirm the cause before buying parts.
Will U0121 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.