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SPN 520 FMI 4 — Service brake air pressure — secondary circuit low
SeriousAbsPower derate likely — plan to stop soon
Applies to: Bendix · Wabco
Service brake air pressure — secondary circuit low.
Don't risk it — get a diesel tech on the truck.
Your SPN/FMI 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
- Low air pressure
- Leak
- Governor issue
- Low air
- Governor
First moves (roadside)
- Check air system pressure
- Inspect for leaks
- Check air system
FAQ
What does SPN 520 FMI 4 mean?
Service brake air pressure — secondary circuit low.
How serious is SPN 520 FMI 4?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of SPN 520 FMI 4?
The most common cause is: Low air pressure. Other possibilities include Leak; Governor issue; Low air; Governor.
Can I keep driving with SPN 520 FMI 4?
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 heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.