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C0121 — ABS Valve Solenoid Fault
SeriousABSPower derate likely — plan to stop soon
Seen on: Heavy Truck
ABS Valve Solenoid Fault. Common causes: ABS modulator valve fault.
Don't risk it — get a diesel tech on the truck.
Look up listed providers near you and give them this code when you call.
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
What does C0121 mean?
ABS Valve Solenoid Fault. Common causes: ABS modulator valve fault.
How serious is C0121?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of C0121?
The most common cause is: ABS modulator valve fault. Other possibilities include Valve failure; contaminated brake fluid.
Can I keep driving with C0121?
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?
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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.