SPN 791 FMI 8 — Left rear wheel speed abnormal
What it means: Left rear wheel speed abnormal.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Inspect
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Oklahoma City, OK
Closest first. Tell them SPN 791 FMI 8 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
Applies to: Bendix · Wabco
What SPN 791 FMI 8 means
Left rear wheel speed abnormal.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Tone ring
- Gap
- Air gap
First moves (roadside)
- Inspect
FAQ
Left rear wheel speed abnormal.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Tone ring. Other possibilities include Gap; Air gap.
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 791 FMI 8, or share the same subsystem.
- Left rear (LF drive) wheel speed erratic (SPN 791 FMI 2)
- 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)
- Right rear wheel speed erratic (SPN 792 FMI 2)
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 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
- Left Front Wheel Speed Sensor Performance (C0034) · Brakes, ABS & Stability
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.