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SPN 791 FMI 8 — Left rear wheel speed abnormal
SeriousAbsPower derate likely — plan to stop soon
Applies to: Bendix · Wabco
Left rear wheel speed abnormal.
Don't risk it — get a diesel tech on the truck.
Look up listed providers near you and give them this SPN/FMI when you call.
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
What does SPN 791 FMI 8 mean?
Left rear wheel speed abnormal.
How serious is SPN 791 FMI 8?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of SPN 791 FMI 8?
The most common cause is: Tone ring. Other possibilities include Gap; Air gap.
Can I keep driving with SPN 791 FMI 8?
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.