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SPN 789 FMI 8 — Left front wheel speed abnormal signal

What it means: Left front wheel speed abnormal signal.

How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.

Check first: Inspect tone ring

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Listed providers nearest Macon, GA

Closest first. Tell them SPN 789 FMI 8 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.

Henry's Truck Repair☎ Call
0.0 mi away — based in Macon, GA · (478) 538-3245
Top Notch Truck and Trailer Repair LLC☎ Call
0.0 mi away — based in Macon, GA · (478) 216-8330
Central breakdown heavy duty towing & repair☎ Call
0.0 mi away — based in Macon, GA · (706) 536-3376

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SeriousBrakes, ABS & StabilityPower derate likely — plan to stop soon

Applies to: Bendix · Wabco

What SPN 789 FMI 8 means

Left front wheel speed abnormal signal.

How serious is it?

Serious. Power derate likely — plan to stop soon.

Most likely causes

Tone ring damage
Sensor air gap
Wheel bearing
Tone ring
Air gap

First moves (roadside)

Inspect tone ring
Set sensor to touch tone ring
Inspect

FAQ

What does SPN 789 FMI 8 mean?

Left front wheel speed abnormal signal.

How serious is SPN 789 FMI 8?

Serious — Power derate likely — plan to stop soon.

What's the most likely cause of SPN 789 FMI 8?

The most common cause is: Tone ring damage. Other possibilities include Sensor air gap; Wheel bearing; Tone ring; Air gap.

Can I keep driving with SPN 789 FMI 8?

Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.

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Related codes

Codes that commonly appear alongside SPN 789 FMI 8, or share the same subsystem.

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

All Brakes, ABS & Stability codes

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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.