C0121 — ABS Valve Solenoid Fault
What it means: ABS Valve Solenoid Fault.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Check brake fluid condition
Don't risk it — look up listed providers near you and give them this code when you call.
Listed providers nearest Oklahoma City, OK
Closest first. Tell them C0121 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 Oklahoma City, OK · (405) 551-3320
- 0.0 mi away — based in Oklahoma City, OK · (405) 682-1362
- 0.0 mi away — based in Oklahoma City, OK · (405) 632-6010
Seen on: Heavy Truck
What C0121 means
ABS Valve Solenoid Fault. Common causes: ABS modulator valve fault.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- ABS modulator valve fault
- Valve failure
- contaminated brake fluid
First moves (roadside)
- Check brake fluid condition
- test valve operation
- replace modulator
FAQ
ABS Valve Solenoid Fault. Common causes: ABS modulator valve fault.
Serious — Power derate likely — plan to stop soon.
The most common cause is: ABS modulator valve fault. Other possibilities include Valve failure; contaminated brake fluid.
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 C0121, or share the same subsystem.
- Service brake air pressure — primary circuit low (SPN 519 FMI 4)
- Service brake air pressure — secondary circuit low (SPN 520 FMI 4)
- Left front wheel speed sensor erratic (SPN 789 FMI 2)
- Left front wheel speed abnormal signal (SPN 789 FMI 8)
- Right front wheel speed sensor erratic (SPN 790 FMI 2)
- Right front wheel speed abnormal (SPN 790 FMI 8)
- Left rear (LF drive) wheel speed erratic (SPN 791 FMI 2)
- Left rear wheel speed abnormal (SPN 791 FMI 8)
Same symptom: ABS or stability lamp
Other codes behind a lit ABS or stability lamp — the ones that turn an inspection into an out-of-service order.
- Right rear wheel speed erratic (SPN 792 FMI 2) · Brakes, ABS & Stability
- Right rear wheel speed abnormal (SPN 792 FMI 8) · Brakes, ABS & Stability
- Left tag/pusher wheel speed erratic (SPN 793 FMI 2) · Brakes, ABS & Stability
- Right tag/pusher wheel speed erratic (SPN 794 FMI 2) · Brakes, ABS & Stability
- ATC (automatic traction control) diff brake — not responding (SPN 974 FMI 7) · Brakes, ABS & Stability
- Left Front Wheel Speed Sensor Circuit Low (C0031) · Brakes, ABS & Stability
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.