SPN 799 FMI 6 — Left rear modulator — short to ground
What it means: Left rear modulator — short to ground.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Disconnect and measure
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Albuquerque, NM
Closest first. Tell them SPN 799 FMI 6 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 Albuquerque, NM · (505) 226-2125
- 0.0 mi away — based in Albuquerque, NM · (505) 823-9696
- 0.0 mi away — based in Albuquerque, NM · (505) 220-3541
Applies to: Bendix · Wabco
What SPN 799 FMI 6 means
Left rear modulator — short to ground.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Short
- Wire ground
First moves (roadside)
- Disconnect and measure
FAQ
Left rear modulator — short to ground.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Short. Other possibilities include Wire ground.
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 799 FMI 6, or share the same subsystem.
- Modulator valve — left rear — open (SPN 799 FMI 5)
- 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)
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.