U0121 — Lost comms with ABS
What it means: Lost comms with ABS.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Check J1939 bus
Don't risk it — look up listed providers near you and give them this code when you call.
Listed providers nearest Phoenix, AZ
Closest first. Tell them U0121 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Phoenix, AZ · (623) 304-4187
- 0.0 mi away — based in Phoenix, AZ · (480) 458-7715
- 0.0 mi away — based in Phoenix, AZ · (480) 530-7347
What U0121 means
Lost comms with ABS. Common causes: J1939 issue.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- J1939 issue
First moves (roadside)
- Check J1939 bus
FAQ
Lost comms with ABS. Common causes: J1939 issue.
Serious — Power derate likely — plan to stop soon.
The most common cause is: J1939 issue.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside U0121, or share the same subsystem.
- ECU Power Supply — Intermittent / Erratic (SPN 627 FMI 2)
- Battery / Charging Voltage — Low (SPN 168 FMI 18)
- Engine Controller (ECM) — Internal Fault (SPN 629 FMI 12)
- Battery voltage low during crank (SPN 168 FMI 17)
- J1939 proprietary datalink lost (SPN 625 FMI 9)
- J1939 Data Link — Communication Error (SPN 639 FMI 2)
- High Speed CAN Bus Fault (U0001)
- Lost Communication with ECM/PCM (U0100)
Same symptom: Lost communication
Other datalink faults. A U-code is a network problem and is diagnosed at the datalink, not at the module that went quiet.
- Lost Communication with Fuel Injector Control Module (U0105) · Electrical, Charging & Network
- Lost Communication with TCM (U0101) · Electrical, Charging & Network
- Lost Communication with Gear Shift Module (U0103) · Electrical, Charging & Network
- Lost Communication with Body Control Module (U0140) · Electrical, Charging & Network
- Invalid Data from ECM (U0401) · Electrical, Charging & Network
- Invalid Data from TCM (U0402) · Electrical, Charging & Network
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.