Home › Fault codes › U0121
U0121 — Lost comms with ABS
SerioustransmissionPower derate likely — plan to stop soon
Lost comms with ABS. Common causes: J1939 issue.
Don't risk it — get a diesel tech on the truck.
Your code rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- J1939 issue
First moves (roadside)
- Check J1939 bus
FAQ
What does U0121 mean?
Lost comms with ABS. Common causes: J1939 issue.
How serious is U0121?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of U0121?
The most common cause is: J1939 issue.
Can I keep driving with U0121?
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Throwing this code right now?
Find a mechanic or related guides
General diagnostic guidance for diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.