SPN 191 FMI 2 — Transmission output shaft speed erratic
What it means: Transmission output shaft speed erratic.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Inspect sensor and tone wheel
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 191 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 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: Allison · Eaton · Volvo · UltraShift PLUS
What SPN 191 FMI 2 means
Transmission output shaft speed erratic.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Speed sensor
- Tone wheel
- Wiring
- Sensor
- Wire
First moves (roadside)
- Inspect sensor and tone wheel
- Inspect sensor
FAQ
Transmission output shaft speed erratic.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Speed sensor. Other possibilities include Tone wheel; Wiring; Sensor; Wire.
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 191 FMI 2, or share the same subsystem.
- Output shaft speed abnormal (SPN 191 FMI 8)
- Output shaft speed not updating (J1939) (SPN 191 FMI 9)
- Main shaft speed erratic (SPN 160 FMI 2)
- Input shaft speed erratic (SPN 161 FMI 2)
- Transmission output retarder speed erratic (SPN 162 FMI 2)
- Trans oil temperature very high (SPN 177 FMI 0)
- Trans oil temperature voltage high (SPN 177 FMI 3)
- Trans oil temperature voltage low (SPN 177 FMI 4)
Same symptom: Won't shift
Other codes that show up as a truck that won't take a gear or drops out of one.
- Transmission output retarder position erratic (SPN 2919 FMI 2) · Drivetrain & Transmission
- Retarder not responding (SPN 2920 FMI 7) · Drivetrain & Transmission
- Gear selection failure (SPN 749 FMI 7) · Drivetrain & Transmission
- Range shift signal erratic (SPN 753 FMI 2) · Drivetrain & Transmission
- Shift finger not moving to commanded position (SPN 787 FMI 7) · Drivetrain & Transmission
- Clutch actuator open circuit (SPN 788 FMI 5) · Drivetrain & Transmission
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.