SPN 789 FMI 2 — Left front wheel speed sensor erratic
What it means: Left front wheel speed sensor erratic.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Push sensor fully to tone ring (press-fit to bearing)
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Indianapolis, IN
Closest first. Tell them SPN 789 FMI 2 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 Indianapolis, IN · (317) 789-5002
- 0.0 mi away — based in Indianapolis, IN · (317) 300-7000
- 0.0 mi away — based in Indianapolis, IN · (317) 646-8208
Applies to: Bendix · Wabco · EC-60 / EC-80 · Wabco/ZF (common on Volvo/Freightliner)
What SPN 789 FMI 2 means
Left front wheel speed sensor erratic.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Sensor pushed away from tone wheel
- Wheel bearing slop
- Corroded sensor
- Damaged tone ring
- Sensor pushed out
- Tone ring damage
- Bearing
First moves (roadside)
- Push sensor fully to tone ring (press-fit to bearing)
- Inspect tone ring for missing teeth
- Resistance check sensor (1000-2500 ohm typical)
- Reset sensor to tone ring
- Inspect tone ring
- Resistance check
Common misdiagnoses
- Replacing sensor when it just needs pushing in
FAQ
Left front wheel speed sensor erratic.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Sensor pushed away from tone wheel. Other possibilities include Wheel bearing slop; Corroded sensor; Damaged tone ring; Sensor pushed out; Tone ring damage; Bearing.
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 789 FMI 2, or share the same subsystem.
- Left front wheel speed abnormal signal (SPN 789 FMI 8)
- Service brake air pressure — primary circuit low (SPN 519 FMI 4)
- Service brake air pressure — secondary circuit low (SPN 520 FMI 4)
- 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)
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.