SPN 520 FMI 4 — Service brake air pressure — secondary circuit low
What it means: Service brake air pressure — secondary circuit low.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Check air system pressure
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Phoenix, AZ
Closest first. Tell them SPN 520 FMI 4 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
Applies to: Bendix · Wabco
What SPN 520 FMI 4 means
Service brake air pressure — secondary circuit low.
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
Service brake air pressure — secondary circuit low.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Low air pressure. Other possibilities include Leak; Governor issue; Low air; Governor.
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 SPN 520 FMI 4, or share the same subsystem.
- Service brake air pressure — primary circuit low (SPN 519 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)
- Right rear wheel speed erratic (SPN 792 FMI 2)
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.